My only prior experience with Street Fighter was 20 minutes with the
Street Fighter II Hyper Fighting Demo on Xbox live arcade 2 days before
SFIV came out. For a laugh, me and my friends went to a SFIV tournament
and got the game at midnight. We then proceeded to play it. A lot. And
finally I feel justified to comment on what the game has to offer.
You see, at the time of writing, Street Fighter IV has been out for
just over a month. And it plays the long game. What I mean by this is
its not akin to something like Call of Duty in design, its not so much
a game that you play, finish and be done with it. In fact, its more
akin to a board game in that it is a blank slate of a game which you
can play over and over
again. Think of it not as a means to and end, but an end to a means. If that makes any sense.

Now, the challenge mode is essentially a drawn out tutorial that teaches you the combos and other such moves you will need to perform to actually win a fight. There are 5 "easy" and 5 "hard" trials and all of these involve a series of combos or moves you will be required to pull off to pass. While this helped me greatly on the ability to perform these moves, there was nothing to indicate when I would want to use them, yet it feels like it shouldn't tell you anyway. Learning from your mistakes, and how others completely wipe the floor with you is something much more rewarding.
As you should already know unless you've been under some elaborate rock for the last century, Street Fighter is a fighting game. And therefore, as with all fighting games, it will live or die on whether there are real people to fight against (even if the AI does a half decent job at imitating them). And by god will it have them. The fighting system is easy to learn and damn hard to master. Once you've been hooked, it's unlikely you'll be able to let go.
For the uninitiated (as I was),'ll tell you how the system works. There are 3 buttons for kicks, 3 buttons for punches (Light/Medium/Heavy) and a directional input. Thats it. And it obviously will work best with an Arcade stick built for that specific use. However, you can easily reach every button at once with a non-default set up (using the shoulder buttons and triggers) and even set the spare 2 buttons to be a button combination. It's very simple to get up and start punching and dodging. From there there is special attacks, blocking, super moves, ultra moves, focus moves, combos, dash cancels, cross-ups etc. etc. Within a month of casual playing I've managed to get a basic hang of all of these. Learning is something you will do quickly.

And beating the other player is what its all about. Even if xbox live is full of people who have a strange urge to send you hate mail all the time, the internet is vast enough that there are many many people who want a nice fun game of street fighter. Unfortunately, your enjoyment will be decreased greatly if there is not that human aspect. For example, people have a tendancy to jump when you charge up a fireball with gouken. If you happen to be unleashing the upwards fireball, that person is in for a shock, and you get a nice endorphine rush. AI never does this, always waits in a more calculated way and is generally more predictable.
There are 25 almost perfectly balanced characters in all, with most of them extremely inventive and unique, save for the one that should be the most unique, inventive and balanced. The boss. Seth is a blank palate who knicks other characters moves. Nothing crazy, nothing unique, just someone who closely resembles Dr. Manhattan from Watchmen but with a strange circle on his chest. Not a fun character to fight against or fight as. Aside from that, all the characters are open to love and hate in a fair way. You will hate some, you will love others, but that is personal choice and good design, not bad cheap bosses.

Yeah, I'll go do that. And you should too.