We've had great luck with them over the years. Our standard deployment is currently the x4u (albeit a different solution powering the handsets as were just now evaluating fusion)

Build quality may not be on par with some of the others but if they are treated well they last just as long as anything else.
Been evaluating an X3 and X4 in the last year with Fusion. Never had any lockups, latest firmwares. Compared with other phones under evaluation from Grandstream and Htek, only issues with the Fanvils has been with screen glare compared to the others, no matter the adjustments. I'd recommend them!