I've played in an all wizard game once.... or twice, actually.
First time I completely dominated as it was around 5th level, and I was playing an optimized blaster, and the other two were more typical save or die and battlefield control types.
And we were fighting undead.
I think the other casters were more focused on working against humanoids or something, because they didn't seem to get much done.

Second game, Tippy was in the party with Cindy.
'nuff said, I think. So it was more like party and Tippy.

I think a good Wizard only game would indeed have different wizards focusing on different things, mostly just to keep it so each person has something they excel and do best, rather than one person dominating more frequently.


As for the Gandalf discussion... I've always thought of Gandalf as a Factotum.

He doesn't cast many spells, but can draw on a huge range without needing a spellbook (just like a Factotum)- he's good at fighting, even though he is an old man (sign of his Int score aiding him all over the place), he seems to know and be able to do anything (lots of skill points...)

So yeah, I think that's probably the best class for him. I can see good outsider, which would fit with his background, but I think class wise, Factotum works best.