The bloodletters of Khorne have lost 1T compared to the previous edition. This is actually a serious detriment to bloodletters and their survivability. To compensate, they've also become plenty cheaper. But with a 5+ invulnerable save, they're frequently not going to make it to their target.
So what use are they? Well, I'm still seeing them as a herald delivery system (or skulltaker delivery system). They are simply there to soak up some wounds and be part of a large Khorne horde. Their attacks are nothing to be ashamed of: AP3 is still powerful and on the charge, they are going to take care of space marines readily.
In terms of options, they can get plenty of extra members for their unit: clearly if playing horde this is a great option. One member can also be upgraded to a bloodreaper, gaining an extra attack. For a small investment of extra points, this is one that should always be taken (unless you're worried about challenges? but you're playing Khorne - you shouldn't be!). They can have lesser and greater rewards as well. As an option, the Axe of Khorne seems appealing as well. The instrument is also a good option here. For a khorne army, the icon is a good idea, as might be the banner of blood for extra charge range.
Here are a few sample builds.
10 bloodletters of Khorne, including 1 bloodreaper with an Axe of Khorne, instrument of chaos (125 points)
A distraction unit. Used as a screen. If it survives, it charges and makes a mess of space marines.
20 bloodletters of Khorne, including 1 bloodreaper with an Axe of Khorne, instrument of chaos, icon: banner of blood (245 points)
The fully upgraded unit. And the price tag is still rather reasonable, even despite the low toughness models.
16 bloodletters of Khorne, including 1 bloodreaper with an Axe of Khorne (175 points)
A herald delivery unit, with a fluffy number of models.