I'm sure it being built up so much didn't help, but I was underwhelmed. A lot of it felt very dated; the science was clunky, and Matheson focused on it too much. Though I didn't find Neville, the protagonist, particularly interesting or likeable, I would much rather have seen more of his personal story than hear so much about his research and theories and etcetera. In fact, that probably would have helped flesh out his character a bit more. The glimpses into his past were the strongest parts, I thought, where we learn about the fate of his daughter and wife in the earlier stages of the epidemic. ...No, actually, I take that back. The strongest part was the dog, which managed to actually elicit an emotional reaction from me. I was rooting for him to pull through, and not because I particularly cared that Neville get a companion out of the deal, but more...because it was a dog. Did Matheson consciously make use of the fact that humans tend to be sympathetic toward animals in order to get readers more involved in the story? I have no clue, and don't particularly care. ♥ Puppieeee.♥
The plotline about the "vampire" epidemic being caused by a virus--eh. I suppose part of it is that what innovation it may have had when first published has lost impact due to being used and recycled in various media, so much so that it doesn't feel particularly original. Also, I think Matheson tried too hard to make traditional vampire traits (aversion to garlic and crosses; sunlight/being staked in the heart=death) work within the whole virus thing, which resulted in some pretty tenuous explanations.
And then there is the ending. Honestly, arriving at the end of the story, the foremost thought in my mind was "that's it?" (though, I am curious now about how it was adapted for the film. The ending especially; it seems like too much of a downer for a Will Smith popcorn flick. [Ur. If it's different, please avoid specifics, though. *hates spoilers*])
I didn't hate it, but didn't particularly like it, either. I'm sure if I hadn't heard so much about it beforehand, if it didn't occupy a niche in SF literature, I would've enjoyed it more--for what it is, it's still an interesting little tale, and worth the read--but hype is unavoidable to a degree, and it definitely failed to live up to that. (This is pretty much exactly the way I felt about Poul Anderson's Three Hearts and Three Lions: decent enough story, and I can see how it was influential, but generally overrated.)
(The edition I read also had a number of Matheson's short stories included, and some of these I probably enjoyed more than Legend, notably: "Buried Talents", an atmospheric Twilight Zone-esque tale of a man who continuously beats a carnival game with unnatural accuracy; "Dress of White Silk", a creepy supernatural story told from the POV of a young girl; and "Witch War", an allegory of war and the arms race, with a wicked bite hidden carefully beneath its deceptively-fluffy façade.)