I tried BTCGuild, and had issues. Loads of idle miner times that cost me hours of mining. Definitely more than a few percent, I would guess around 10%, but could have been way more.
If you have multiple miners running on each video card at all times the idle times are meaningless. I mine two pools concurrently so I don't have these issues, while not endangering the relevance of the currency that I am attempting to acquire.