I buy too many models that I like leaving me with a heap of half completed armies.
I do however like Eldar, they're a great army and can be specialised and characterised in so many different ways, such as Aspect heavy (swordwind?), mechanised, guardian/psyker based (ulthwe), full jet bikes (saim-hann), corsairs, exodites or even spirit based (iyanden). the most appealing factor is that they're so specialised and strategic that they seem like they'd be a blast to play as you need to think carefully about what everything is doing.
If I was to start 40k again, I'd definitely collect Eldar, and am even thinking about a small jetbike force...