For Instance I want to spend exactly $50.00 buying at $13.7; they should then tell me exactly how many bitcoins that represents after taking out their commission.
So far I have failed to get it exactly right on three different exchanges, and arithmetic is supposed to be simple.
I would really like that. The user interface is sill very rough in almost every site... Camp BX's interface has a few advanced options, but lacks a simple feature like this one. I hope that it will improve with time. So for now I am only considering two factors to choose an exchange (the interface is not an option because it sucks everywhere): security and easy ways to deposit/withdraw.