Performance

It is extremely difficult to get 100% performance out of these devices. The Teraflop ratings they are given are for an ideal situation where the alu's are fully occupied across all threads and the max mem bandwith is being achieved.

The class of problems which can get you to this level of performance is rather small...

However for their available processing power they use a substantially lower amount of power than a cluster of cpu's, but are admittedly harder to program efficiently.