I've used one of the HP branded models and my results were not so good. The maximum resolution is 1680x1050 which is too low for some 23" and most 24" displays.

I've used one of the HP branded models and my results were not so good. The maximum resolution is 1680x1050 which is too low for some 23" and most 24" displays. The high CPU usage it demanded was too much for a 1.8Ghz MacBook Air Rev C to handle without significant lagging and beach balling. The performance was acceptable on a MBP CD2 2Ghz for the most part but spikes in CPU usage (Vmware for example) would cause some wonky things to occur on the USB display. It also introduced some sleep/wake problems which may have since been resolved with a driver update. I think USB2 is too much of a bottleneck for this type of device to offer a seamless solution. Unless you need a third display on an iMac, Mini or MacBook you're better off buying $60 worth of spare adapters IMO.