The Daemonic legions of the Blood God are traditionally shock and awe troops; drop them in and decimate the enemy to either reduce their effectiveness as to render them non-threatening or to completely eliminate them before they are able to do any damage. Shock and awe as a military tactic can be dated as far back as Sun Tzu's "The Art of War" and it's possible that it goes back further than that. This is the basis of all of Khorne's units and in this area they do a pretty damn good job.
Looking at the basic profile of the Bloodletter we can immediately see that they have the highest WS and BS of the whole troops section although they share their WS value with the Daemonettes. Their WS and BS values are both 5 which means that their Hellblades (more on those later) will hit anything with a WS of 4 or less on a D6 roll of 3+ and will hit anything with a WS value of 5 or more on a 4+ so immediately they have a pretty high hitting potential. The trade off for this potential is that they are the most expensive of the Troops options in the Codex coming in at 100 points for a basic 10 man unit. This seems expensive on the face of it but when you examine their supporting rules they actually become quite the bargain; firstly they are Daemons of Khorne and get the corresponding special rule. This rule grants Bloodletters the Furious Charge special rule that grants +1 strength if the Bloodletters make a successful charge pushing them from s4 to s5. Against most things you're coming up against you're either rolling to wound on 2+ or 3+ and anything that's making you roll higher than that to wound is probably not something that the Bloodletters are suitable for anyway. This strength boost is nice on its own but when added to the fact that each MODEL (not unit) gets an additional attack when making a charge you're getting 20 s5 attacks that are hitting on 3+/4+ and wounding on 2+/3+ and this is all before adding any upgrades of any kind.
While not strictly an upgrade I want to go through the standard issue weapon of the Bloodletters before going any further. I've mentioned the Hellblade before but here's the deal; it's a Melee weapon (shock horror) that uses the strength value of the user and has an AP value of 3. This seems like an unimpressive weapon especially considering that it used to have an AP value of 2 (a move that many in the hobby have said has nerfed the weapon) but this edition seems to have changed the use of these Daemons from being able to kill everything and anything to taking on things more within their range. You have to admit that a Troop choice being able to handle the toughest Elite and Heavy Support choices isn't exactly promoting balance so now it looks like they're focused on handling troops and maybe light vehicles. Combine this with the things mentioned in the last paragraph (increased strength and attacks) and it is still a pretty terrifying weapon. So now that the Hellblade is covered, onto the actual upgrades.
As with most units there are three initial upgrades that can be taken; unit champion, musician and standard bearer. Now you'd be forgiven for thinking that I'd slipped into Fantasy Battle talk but no this is still 40k I promise. We'll start with the instrument as the benefits take the least time to go through. I'm assuming that anyone reading this will already know about the whole Daemonic Deep Striking rule and that the Icon and the Instrument alter how Deep Striking happens so I'll continue on this assumption. The Instrument of Chaos has two effects that are beneficial to the army as a whole; the first is that it allows a unit still in reserve to effectively piggy-back onto the battlefield when another unit with an instrument Deep Strikes in so you get two units for one Deep Strike roll. This reduces the chance that units will have to roll on the Mishap table and as I understand it if the unit that is Deep Striking does roll for a Mishap then it doesn't affect the unit making the piggy-back (though I may be wrong on this, the latest FAQ really wasn't very helpful at all). The other benefit of an Instrument is also the one that is talked about the least, even though I think it is even more beneficial; each Instrument of Chaos held by a Khorne unit allows one dice to be re-rolled when the Warp Storm table result is Khorne's Wrath. What this means is that if you have an Instrument in with your Bloodletters and you roll Khorne's Wrath on the Warp Storm table you have a better chance of keeping those Seekers that have just lined themselves up for a flank charge against the enemy Terminators than you had before. Personally I think the chance to keep a unit alive is seriously worth the 10 points that an Instrument costs and it can really demoralise your opponent when he/she is celebrating one of your units being killed only to watch in horror as you re-roll the dice and it turns out that they're not actually checking out at all.
The Icon, as with the Instrument, carries with it a bonus to the Daemon Deep Striking rule in that it either reduces or eliminates Scatter for units from the Codex. For example a unit of Bloodletters with an Icon allows for any other unit with the Daemon special rule that Deep Strikes within 6" to roll only one D6 when rolling for scatter distance. If that unit happens to be a unit with the Daemon of Khorne special rule then it doesn't roll to scatter at all and instead comes in on target automatically. Another interesting bonus to the Icon that it adds 1 to the combat resolution result if there is another unit in the same combat that has another Icon. This raises some very interesting opportunities for combinations and the first one that comes to mind is with the Daemonettes but I think unit combinations will have to be another blog because I could go on forever. The Icon carried by the Bloodletters can also be upgraded to the Banner of Blood which not only carries the benefits of the Icon but also allows for an additional once-per-game effect. Each God has an Icon upgrade that grants a one use effect but the upgrade for Khorne allows for the charge range of the unit carrying the Icon to charge 6+D6" instead of 2D6" and the use of this must be declared before rolling so you can't rescue a terrible charge roll unfortunately.
The Bloodreaper is the unit leader of the Bloodletters and has the exact same profile as the Bloodletter just with one extra attack. Adding that to a basic unit gets you 11 attacks at s4. On a charge that works out at 22 attacks at s5 which isn't too shabby and for a low, low price of 5 points. I guess the main advantage of the Bloodreaper is that it is able to take 20 points of Lesser and Greater Gifts. What this means is that the Bloodreaper is guaranteed to be able to get an Etherblade (10 points, AP2, master crafted and specialist), an Axe of Khorne (10 points, AP2, Instant Death on all to wound rolls of 6, specialist), a Greater Etherblade (20 points, AP2, +1 strength, master crafted, specialist) or a Blade of Blood (AP2, Unwieldy, Rampage, Specialist). These weapons are all pretty great and the other possibilities on the rewards table are also pretty good but the question has to be asked whether these upgrades are worth it or whether the points can be spent elsewhere. It may seem like 20 points isn't a lot but if those 20 points are spent on each of the unit champions then those points are taken from another unit.
Aside from all of these points there a couple of other things that are worth mentioning before I wrap this up. Firstly is that the Bloodletters are the only Troop choice with an Armour Save, although this is 6+ and is therefore unlikely to be used very often considering that the Invulnerable Save is 5+ and can be used more often anyway. Another thing is that the Initiative value of the Bloodletters is 4 which is the same as most units in a generic Space Marine army so they will strike at the same time as most common enemies.
So what we have is a very powerful Hammer unit; they're designed to hit hard, hit fast and not leave much behind if anything. Now they are likely to get shot at and they are likely to get killed because the Khorne name commands a lot of respect and fear from non-Daemon players that know exactly how much of a bad idea it is to take these guys on so running them in large units is advisable not only for survival purposes but also for hard hitting purposes. A unit of 20 with a Bloodreaper gets 21 attacks which doubles to 42 s5 AP3 attacks when the unit charges so you can see why they're going to be the focus of a lot of fire. They may have been balanced out under the new Codex but make no mistake this unit has not lost any of its edge and that edge happens to be razor sharp; all the better for harvesting skulls for the skull throne!