I play and 11th level halfling (Rgr 1, Rog 9, Dungeon Delver 1), with weapon finessed +1 daggers and a +2 crossbow of speed. The Sorceror has taken Improved Invisibility as one of his few spells and we make great use of the combo.
And I can't believe Ice Bear that no one has played a Rogue in your campaign - great all round character - particularly with Use Magic Device Skill (although my halfling's 9 Cha hurts that).
Anyway, my observations about Sneak Attack are that even though I get 3 attacks for a full attack, the risks in moving into a flank position aren't always worth it. And as Phule said, not everything is subject to critical hits. Mind you, occassionally it really shocks the DM when he does cut lose with 3d4+5+15d6 damage for a round. Got meself a huge red dragon with it last week (don't ask).
Nowadays I mostly rely on Improved Invisibility and the crossbow. But overall, doesn't unbalance the game at all - and I'm a very tactical player.