Basic Oni stuff:
Just in case you didn't know, his air dash can go upwards or forwards as well if you use different punches.
The EX version of his qcf + kick slashing move is a cross up and is hard to react to, particularly online. You can juggle straight into either of his ultras off it or just a dragon punch if you don't have ultra.
You can do his super in the air as well as on the ground.
Ultra 1 (the fireball one) has three different versions. There's the regular, one that goes straight upwards and it can also be done in the air.
He also has two versions of each of his normal attacks when standing up, so there's jab or forward + jab, medium punch or forward + medium punch. A lot of characters have one or two of these but Oni has all six. The forward + hard punch one can absorb fireballs, which is pretty pimp.
One of his major weaknesses is that you can't FADC his dragon punch unless it hits the opponent, so if it gets blocked you're screwed. It's literally the only uppercut/dragon punch-esque move in the game with that property. Guess it makes him less shoto-y. Although he has some of the same basic tools I really don't think he plays like a shoto at all.
Balance-wise, the short version is this:
Yun is the best.
Makoto, Fei Long and Yang are one step below him
Everyone else slots in beneath them to varying degrees, though nobody is completely rubbish any more.
One line better or worse summaries. I can be specific if there's any you're interested in:
Ryu got worse.
Ken got much better.
Chun Li is almost unchanged.
Blanka got worse for shit players but better for good ones.
Zangief got worse but he does alright against the twins.
E. Honda got worse.
Guile got worse.
Dhalsim got worse.
Balrog got a little bit worse.
Vega got better.
Sagat got better.
Bison got a little bit worse.
Akuma is almost unchanged.
Fei Long got much better despite being one of the best in SSFIV anyway, which is why he's top tier now.
T. Hawk got better.
Dee jay got much, much better.
Cammy got changed fairly significantly and it's still 50/50 on whether she's better or worse.
C. Viper stayed almost the same and is now upper tier simply because most of her rivals and bad match-up characters got worse.
Abel got a little worse.
Fuerte got much worse.
Rufus got a little worse.
Hakan got much, much better.
Juri got a little better.
Rose got worse.
Gen got much better.
Sakura got much better
Dan got much, much better.
Cody got better.
Guy got better.
Dudley got better.
Ibuki is roughly the same.
Makoto got much, much, much, much etc etc etc. better.
Adon changed a little but isn't really better or worse.
Gouken got much, much better.
Seth changed a fair amount and is possibly a little bit better for it. Debatable.
For balance changes, there is no complete list but Shoryuken and Eventhubs have compiled most of the known changes. There's also an ongoing video series detailing them here:
http://www.youtube.com/user/GLHFtv
For matches and replays:
This is the best channel for SFIV replays from online matches:
http://www.youtube.com/user/0ShinAkuma0
Search his uploads and check out some of his playlists. If you see anyone on there who impresses you you can get their gamertag from the description or video and then follow them.
Yun footage is easy because he's the best character in the game and is currently played by arguably the best player in the world, Daigo Umehara. His tag on XBL is gameinn. A lot of the top Japanese tournament players at least have Yun as a second or third character now because you can't really compete without it.
Yang is pretty popular as well, but I'd say Nemo is probably the best. His XBL tag is nmot.
For both of the twins you need to look to Japan for the best stuff because they've had AE for almost a year now, though there are already some beastly American Yun players and Justin Wong and Australia's HumanBomb both have mighty Yangs.
Oni is a second character for a lot of good players but I don't think anyone is using him as their tournament "main" that I've seen. One of Japan's top Akuma players, Eita, uses Oni on Xbox live though. His tag is Eita1988.
Evil Ryu I don't know. He's not taking off really. He's a bit like Ryu and a bit like Akuma but not as good as either of them because he doesn't quite have Ryu's solid fundamentals and he doesn't have Akuma's ridiculous array of offensive and escape options. He's good fun, I like his Third Strike styled normal attacks and his new special moves and combos and such, but I can see why competitive players aren't using him. I love that you can cancel his forward + mk hop kick straight into raging demon. It means you can hop over someone's low attack and immediately punish it with Demon. Looks bad ass. His EX axe kick is really good too and is an overhead.
This match from NorCal Regionals 9 where America's best Tekken player rocked up with his Evil Ryu and upset Justin Wong is worth a watch:
http://www.youtube.com/watch?v=isvqLaQ1jRE