Instawallet is an intermediary like a mixing service, but nowhere near as safe. Some people believe it's enough to offer plausible deniability, ie, someone follows the bitcoin trail back from your SR address to an Instawallet address to the exchange. The exchange knows who you are but you can claim that you sent the coins to someone else, and as long as you access Instawallet over Tor, nobody can prove it's you. I'm not particularly convinced of that method. I'd rather not be identified in the first place, which is why if I bought coins in a non-anonymous way, I'd properly mix them.