I'm using a DMPC right now, since it's rotating DM, we're all technically DMPCs occasionally. I find in general, the DMPC should be more a support role. For the poll, I chose everything except striker, including other. It's also important to avoid specializing entirely in areas where it would become a "self conversation." Which 4E roles don't have a definition for. In 3E, a rogue is a terrible choice for a DMPC, as is any party "face." The DMPC should not be the one who has to be responsible and check for traps. Or the one doing the talking for the party. I even hesitate to make a "pokedex" (our group's term) DMPC, always insisting if I have one to make the other PCs try the knowledge checks first, and only roll mine if they didn't learn anything or enough to adequately deal with the situation. Any DMPC that's the party scout is also clearly a disaster waiting to happen.
Generally, a healer and buffer is the best route to go, though an arcane caster focused on battlefield control can also work. I'd avoid flashy evocations or save-or-lose spells in that case, though. A caster like sorcerer with an unchanging list of spells is preffered, though a prepared caster can work if you make sure to lay out the spells prepared and stick to them, so you clearly weren't abusing your "knowledge" of what the upcoming encounters would be.
Anything like a typical barbarian is just plain out, though. An atypical barbarian, maybe one that traded rage for ranged weapon feats (UA) that sets up flanks for the rogue and maxes intimidate to control the frontline could work, though.
It's definitely a tough balancing act, but I'm always surprised at how abominable people here find the situation. I've never seen players suspect the DM with his own PC of being unfair IME. It's just a matter of conflict of interest (including percieved conflict of interest), and keeping conscious of it while still keeping the DMPC useful. The only thing worse than an uber DMPC is a dead weight sponging off xp and treasure.