These others have covered all the main points, so I'll give the gratuitous anecdote.
In my first 3E campaign, I played a Sorcerer, and another guy in the group played a Wizard. Basically, we worked great together. He blasted, I did the utility stuff and nondamage spells. (Yes, that's backwards from how you'd expect it to work.)
Now, speaking from that, I can say that playing a Sorcerer can get boring if you're not careful. You have to pick useful non-combat spells, the sort of thing where you need to cast several of the same spell but don't know if you'll need them. For example, I took Comprehend Languages, Clairvoyance, Fabricate, and Teleport. Each of these is something a Wizard isn't likely to waste a slot on (well, except Teleport) but that can be really useful on a moment's notice often enough that you can't just make up for it with scrolls and wands.
That being said, when we got the Psionics Handbook I retroactively switched him to a Shaper Psion. It fit more with the character concept, and it was nice to have at least a FEW spells the other classes couldn't cast. The Psion class has its own flaws (although adding the Mind's Eye and/or ITCK solves a lot of these)
And that's the basic problem. Between Clerics (with all their domains) and Wizards there's already a lack of a niche for them, and if you add Psion to the mix there's really too much overlap. People who want Fireballs go Wizard, those that want spontaneous casting go Psion, and the munchkins go Cleric.
At that point, the only distinguishing feature of the Sorcerer is its spells/day, and that isn't quite as useful as you'd think. After all, the group will stop to rest when the Cleric, Druid, and Wizard run low on spells, even if the Sorcerer has a bunch left. What it does is let the Sorcerer cast spells without worry.
So, IMC, we eventually combined the Wizard and Sorcerer into one class. But, YMMV.