I think Smith nailed his part, and showed Neville's slowly-deteriorating psyche perfectly, in a way that made me emotionally invested in him. I thought it was pitch-perfect (minus "zombie" effects) until he tried to attack them/kill himself on the pier, at which point it got ... hmm ... I'm not sure how to put it ... not so much "cliched," as ... well, I thought it was original until then anyway, then it seemed to borrow a lot from other movies.
And the one thing I *hated* about it was how they turned it into a religious allegory in the last 15-20 minutes, when it had NOTHING whatsoever to do with that at all until that point. I would have had no problem with it if it had been a theme throughout the film, but not randomly at the end.
Although I must say, the "Please say 'hello' to me" scene was probably my favorite emotional scene in any film I saw all year - it actually made me ache with sympathy for Neville, and Smith did an amazing job with that scene alone.
I'll make a fan-edit of it as a short film where the "dummy" he set out at the pier was actually him - just end it with the POV shot of the vampires/zombies lunging at him, and cut to credits, no music or sound. Depressing, yeah, but I think it would be intense. Dunno if it would make it better, but I'd like it more.