The only thing about it that I really dislike is that there's no option to delete or cancel your account (a trend with startups). I don't fall to the silly excuse that it's in beta or that they'll have such options "soon". This should be there since day 1. Shows respect for the user/tester and sends the message that your information/data is yours to keep/remove whenever you want.

Keep in mind that this is not a criticism towards Prismatic (which is really cool and well done), I just used them as an example to a common startup practice.