In three different one or two person game (all with the same DM), I've played a conjurer, druid, and dread necromancer. All of them were semi-self nerfed just because I knew there were some things he just flat-out wouldn't be able to handle.
Conjurer
This was in the two-player game, with a Hexblade. I never took Alter Self or Polymorph, just so I wouldn't outshine him in melee. I seldom used damage-dealing spells when I got to a high enough level that I could, just so he could do the killing. I did tie stuff up mercilessly with Glitterdust, Web, Grease, Stinking Cloud, Black Tentacles, Solid Fog, and the like.
So, I never pulled out all the stops, but I basically maintained enough versatility to make most encounters fairly easy.
Druid
I wanted to try a Shapeshifting VoP druid for the helluvit. It actually worked out pretty well. At low levels, I was quite self-sufficient. At high levels, I had enough power with my spells alone that I was doing just fine. Like the last character, I didn't have huge amounts of vertical power, but I could do about anything I wanted with a bit of creativity.
Dread Necro
This one was awesome through a combination of spells, PrCs, and magic items. I pulled off Scry & Die more than once. I built a few undead murder machines and carried them around in an Enveloping Pit with a scaffolding built in. This one was the most powerful by virtue of getting to the highest level.
At the end of the day, there was a lot of stuff I just didn't do because I knew it would piss the DM off. As it was, he had a hard time keeping tabs on everything I could do. The most egregious example I can think of was with my dread necro: he put me up against a dragon that had dread necro levels and five hidden huge skeletons. I opened up with two Destructions (Belt of Battle) and the dragon rolled poorly enough to die on the second one. At the time, I didn't know he had dread necro levels or I wouldn't have tried this, fearing for Death Ward. Sadly, the DM totally forgot about that spell despite me casting it daily for the past several sessions.