As far as pricing goes, you're charged for the wall clock time spent running inside the zone, so if your task queues up because the system is jammed, you're not paying for that. Task runtime can be affected on busy systems, but this has been in the noise for the jobs we've looked at. We're also looking to see how the system gets used in order to understand whether it would be useful to have other pricing options (e.g., tiers of service with guaranteed resources).
As for performance: we're using the system internally every day, and we've found the performance very good. Of course, the best way to find out is to evaluate it on your own workload. :)