I buy from Smashwords. They are convenient for me for getting books for reading on my iPod (Stanza) or my phone (Aldiko). It's handy that I can choose to put the books onto my Reader as well.
(Or they were, considering that I just got a kindle this week and may start using the kindle apps more.)
I do tend to only pick books based on active threads here, and I haven't had anything but good experiences from the few books that I have read. (And I should go into the corresponding threads and say so, but I haven't because those threads tend to be several months since their last post and I don't want to bump just to say I liked the book.
)