SF4's about as good of a competitive fighter as we've seen. mechanics work, balance is pretty amazing, game has a lot of hype. not as much as Marvel, but there are some pretty incredible moments.
I think it's solid. Balance especially is something it has over the rest of the series for sure. If I were making a couple changes to SF4 I'd remove ultras, increase walk speed, and buff throws. I do think it's the best competitive FG being played right now in the community. Skullgirls and KOF get more love as "true honest fighting games no comeback mechanics" but SF4 has a better mixture of accessibility, being entertaining to watch, etc.
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http://www.youtube.com/watch?v=npHDxSvwCE0