yeah its not bad. I had a couple of issues with the early beta drivers, but the new drivers available now from DisplayLink appear to work better. For general work, web design and video it works great. You do notice lag when the screen saver is on, it doesn't cope well with 3D, but for everything else it works great.
When I upgraded to snow leopard, I had to reinstall the new DisplayLink beta driver, but now it works fine.