I hear a lot of distain for incubi, but I still stand by mine. I think they were better in the oldbook where you could reliably hand them (4+) Feel No Pain and quickly get them furious charge, but they're still pretty scary. As has been pointed out, they're ideally useful against power armored targets that aren't as scary when hitting back. They are "expensive," but not for what they do. A squad of 5 with a klaivex wielding demi klaives does something like...
3 attacks each for normal guys on the charge = 12 attacks, probably hitting on 3s, so 8 hits, assuming marine equivalents and no furious charge you wound on 4s for 4 wounds meaning 4 dead marines. And then let's add in the klaivex who will get at least 4 more attacks on the charge, hits with about 2 or 3 of those, does 1 or 2 wounds. So all together, that's ~5.5 dead marines. Now let's say there were at least six marines in that squad thus causing rampage to kick in. The klaivex will get an average of 2 more attacks, 1-2 will hit, so about .5 or 1 wounds for a total of ~6 or ~7 dead marines.
If I'm remembering marine costs correctly, that's around 100 points of marines killed on charge by a little over 120 points of incubi. With a challelnge, the klaivex can ensure that the guy with a power weapon/fist died during that charge, and then you'll kill the remainder on your opponent's turn. Not too shabby. Incubi are all about bullying units that don't want to deal with what you're sending their way. If your target has a bunch of storm shields, you may want to consider a different target. Then again, I still advocate the use of wyches, so take what I say with a grain of salt. >_>
P.S. Just remembered the strength bonus from demiklaives, so the klaivex is doing something like 2 wounds without rampage (instead of 1.5) and about 1 wound with rampage (instead of .75).