I've heard that the Huanyang VFD's tend to break after a certain amount of time, or that the keypad fails. I modified the cooling on mine (well, milled a bit off the heatsink so that it's not really close to the capacitors) and it's been fine for a fairly long time. If it breaks I'll try and fix it and if still nothing but a new one ... not sure weather I'd get the same or not. Depends how long it lasts I suppose.

As Jazz says, ones with vector current control are the best, but naturally they're also more expensive. If you intend to use the spindle a lot at low speed you might be better off making a brushless motor spindle as that will have a lot more torque in that range.

I do have a Siemens 250W (and possibly 550W, shame it's not more) VFD that I don't really need... so if anyone can be bothered to interpret their manual...