I was really excited to give this a try, but I have to say that the sign up process and the upgrade to premium were both a rather trying experience. First, I ended up with a confusing situation where I had two accounts - not that big a deal, but a minor pain.
The real issue came when signing up for premium. First, requiring me to have a PayPal account is bad form - please provide some alternative to this. But, I decided to go ahead anyway, I got the subscription set up through PayPal, and was redirected to cloud 9 where I was supposed to have access to these awesome paid features, right? Nope. It has now been two hours and I'm still getting a "we're waiting for confirmation from the payment provider". Paying for something and not getting instant access is really poor UX - I hope you guys will see this and take it as constructive criticism.
Looking forward to finally giving it a try.