I'm a huge fan of Super Street Fighter IV and already own it on the Xbox 360 and the iPhone. So, originally I was unsure whether I needed another version but after watching preview videos, I was impressed with the graphical detail and smoothness of the game and decided to go for it.
And I've been glad with my purchase so far decision!
The main selling point of course is the 3D visuals, it adds nothing at all to the gameplay, but it just looks really cool to see my favourite characters and backgrounds in 3D.
However, if I'm in a frantic online battle then I will usually turn the 3D off (with a quick flick down of the 3d slider) as the 3d visuals will probably get distorted as I mash the buttons and shake the console.
There is a new "over-the-shoulder" viewpoint which you can play the game in, but I tried it for a couple of minutes and then decided not to use it again. I suppose that the viewpoint helps exaggerates the 3D even more, but it makes the game hard to play as your view is more limited.
The 3DS version has all the characters and levels, as well as the trials. You also earn icons and tags which you can set against your character, which are then visible to other players when you go for online fighting (this will be familiar to people who already have the Xbox or PS3 version).
It has an online mode which works well. I spent 3 hours playing online, and had to stop when the battery light started flashing red. If you want to fight online then the game will match you against another unknown player (or a friend, if you prefer). It doesn't have any online tournament modes, which is a slight pity, but not a big deal.
The other major factor is the controls. On the Xbox 360, I play with an arcade stick, so the controls are really fluid. On the 3DS, it's a bit awkward. You can use either the analogue control or the D-pad for directional controls, but I found the D-pad too awkwardly placed (too close to the bottom-left, so it starts hurting your hand quickly). I use the analogue pad to play, it is a bit imprecise, i.e. at times, when you want to jump forwards or do a ducking block quickly, you will press the wrong direction.
In fact, if I had to rely on the traditional way to do the special moves, let alone the super or ultra moves, then I think the game would be quite unplayable.
Luckily, the game is by default set to "lite" control mode. This means that the bottom touch screen is split into 4 virtual buttons, by default, 2 of the buttons will automatically do special moves, one will trigger that character's super move and the other his ultra move.
Some purists probably won't like this, and may think it's not really in the original spirit of the game. Myself, I have no problem with it, and think that as everyone can use these controls then it is fair.
It is possible to set the game to "pro" control mode, where the virtual buttons can only be mapped to normal buttons, e.g. a punch, kick, or combination of punch or kick buttons. And also you can specify when you play online that you only want to play other "pro" users. But I suspect that the majority of online games will be in "lite" mode as this is the default setting.
One major point about the "lite" mode is that it allows moves that were impossible before, e.g. several characters perform their special moves by having to charge up the attack first, e.g. Guile's sonic boom - hold back for 2 seconds, then forwards and punch, but when these moves are mapped to the virtual buttons they can be performed straightaway without any charging time, e.g. this allows Guile to throw a sonic boom whilst walking forwards (previously impossible), likewise you could do a standing punch comboed into Guile's flash kick.
In my opinion, this makes charge-based characters a lot more powerful, and there doesn't appear to be any balance compensation, i.e. to weaken charge-based characters otherwise, or make other characters more powerful. I don't mind this because I mostly play charge-based characters, but it's something to bear in mind if you are a Ryu/Ken etc fan.
So my final verdict is 4 stars. I find the game a lot of fun. It has all the most important features of it's bigger brother console versions, but I have to deduct a star for the controls, the developers have done well by adding the virtual buttons but the 3DS's physical controls do become a limitation since this game requires a high degree of precision when played at a high level.