At first i hated the idea of Deoxys being in Brawl as a PC. He seems too...unobtainable? I mean, I've not played a pokemon game since Red and Blue, but I do know that in order to get him, you would have had to attend a special pokemon event to download him, or trade with someone who somewhere down the line got a Deoxys. I think there are plenty of other Pokemon that represent the series better, or even just a certain installment / generation of the series. Besides, since Deoxys cannot be found or caught in any of the actual pokemon games, doesn't it mean that he has no specific game to actually represent? Just a thought, i don't actually know that one.
Anyways, now that I've looked him up, and pretty much ignored everyone else's post out of self-admitted laziness as I don't want to read lik 25 or so pages, I have decided that I don't think deoxys would be a bad idea, actually. he seems to have potential for for being a very original addition to the roster. Each of his "B" moves could be to transform into each of his different forms (Normal, Defense, Attack, Speed) and maybe he could have an ^B in additon or something. But anyways, while he is in each of his unique forms, his "A" moves will vary, depending on which form he is in.
For instance, if he tranforms into his "speed" form, he could have very fast "A" attacks and he could move a lot faster, but he could do less damage. For his "defense" form, his movement and "A" moves could be slow, but knocking him back would be rather difficult. his "attack" form could give him strong attacks, but he could go flying more easily or take more damage when he is hit.
Otherwise, if this isn't the case, I am sure they could just make each of the forms into a sort of attack, like he could quickly morph into his attack form and perform a move and then go back to normal form. i dunno, I'm sorta just thinking out loud / in text here.