Temporary site troubles aside, I have used and currently use Havelock and find it has the combination of the best interface and the least "fluff" in terms of securities.

Can also vouch for lightbox, owner/operator of Havelock. He is the most accessible of the 3 exchange operators - almost always available on IRC to chat, or you can even contact him by phone. I've also completed a larger transaction successfully with him on something unrelated to Havelock.