It's great for manual testing, just be aware that the performance of the machines they host the browsers on is REALLY bad, to the point that it may make it hard to do testing of HTML5 apps. For basic website testing it works pretty great.
The application is in some sense basically a wrapper around a VNC connection. It's likely that the performance issues you're seeing are due to VNC latency rather than the power of the virtual machines themselves or the hardware they run on (which is pretty beefy). But your point is well made: given this architecture you can't really watch video or do things that require high framerates.
