Being as guilty of restart-itis as many of the forum-goers here, I've noticed a trend: I get bored and restart when I'm running a full party.
Four, on the other hand, seems to be about optimal for me. This can be a little awkward because inevitably there are a lot of items I can't put to good use, but when I run six, a couple of my characters end up being unimpressive. Plus, divine casters get ... less enticing when a party's Bard acquires War Chant of Sith, at least for me.
With four, all of them can shine in their own way. For example, right now I'm running a Barbarian, Swashbuckler, Fighter/Mage/Cleric and Sorcerer. Sort of wishing I hadn't made the FMC a fighter at times due to slow level progression, but it's a pretty fun and effective combo as these things go.
What about y'all?