I've used bidsnipers, yes. Only once, though, when I was living out in Spain. With a thunderstorm on the way that was guaranteed to wipe out my internet connection, and an item I absolutely wanted to win... so I registered with one of them (can't remember which now). Won the item, which was nice, but within days my eBay account had been hacked & was being used to sell non-existant (obviously) video box sets. I got lucky, someone asked a question before any of the items finished, so I could fix the problem.
Since then, I've decided I simply don't trust any 3rd party sniper tools. And besides, I _like_ the frisson of fear you get sticking a last second bid on... I've lost a few items along the way due to mis-timed clicks, the infuriating "You need to log in" message you sometimes see instead of "your bid has been accepted"... but, that's life.
Mostly, if an item ends between 2am & 7am, I just thow my maximum bid on it, and see what happens.