Update circa Feb 2017: PLEASE READ!

It's been brought to my attention that there is a code for unlocking all the characters and stages in the game. One of the main reasons I could not recommend this game at all on release was that, as far as I knew, it was not possible to unlock all single-player content without multiplayer modes (versus), which requires an extra controller. As this is NOT the case, I can recommend this game if it is on sale (50% off or more, and it has been 75% off previously) and you look up the code. If it matters, you will still need to fufill normal unlock conditions for achievements.

The game still sorely needs an online mode, has no AI for versus, and is not quite a pick-up and play title. The two times I've returned to this game after a long absence, it has taken me a while to relearn the controls, an issue I don't frequently have with other games. This may not be a problem if you play often enough, or with a controller.

My outdated review can be seen below
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If you can gather 1 to 3 people for local multiplayer, then feel free to buy this game.

However, if you plan to play this game by yourself, make sure you have a spare controller before purchasing this game.

Why? Because even with a keyboard, a controller is required to play Versus mode, and certain content (at least 3 map sets and all 4 extra characters) can not be unlocked unless you can access Versus mode.

If you do not have a controller, you will not be able to unlock all single player content.
If you do not have a controller, you will not be able to unlock all the content in a game you paid for.

I would change my stance on this game in the future if either:
1) The ability for a single player to fight against the AI in "versus mode" is added
or
2) An online mulitplater mode is added
The second option may be a little unreasonable, but the first isn't.

Either of these would allow players to play versus mode without a controller, thus giving them the ability to unlock game content that is currently unaccessible with just a keyboard.