Shadowdancers

[FireLance]

I generally agree that getting a sneak attack by spending a move action isn't all that unbalanced, but I'd just like to point out one thing.

The bluff skill and a single feat (a paltry sum compared to the requirements and penalties of shadowdancer) already do that, and they're opposed by a less common skill (sense motive instead of spot).

The defender gets to add his BAB to his Sense Motive check, though, which significantly reduces the effectiveness of this tactic.

Instead, I'd compare HiPS to flanking.