I bought the LFA last night and the LP670-4 the night before. I've yet to use either if them in races (my b-spec drivers have used the Lambo though), but in terms of sheer driving fun in the time trials I've been doing, I'm having more fun in the LFA - and I also love the interior and the sound more than the Lambo.
The 458 Italia was one of the first supercars I bought - I gotta say that I don't really care all that much for it anymore. Sure, it's fast (upgraded to around 650 bhp) and handles well, but unlike most cars its looks haven't been growing on me but instead the opposite (used to think it was beautiful, but not so much anymore), and I've never been fond of its engine sound in this game, regardless of exhaust upgrades.
So far my overall favourite car has probably been my chrome Audi R8 V10, tuned to 835 bhp (still a few upgrades I haven't installed if I recall correctly). In terms of lap times it can't quite match some of the best supercars though, even when they are slightly down on power in comparison.