Lots of people in the diy wood router forum use them. Overall they're good and are a good value for the money in my opinion. The drawbacks are that they can squash stuff onto the CRS rail, but that can be prevented by using scrapers or shields. That may be less of an issue with a plasma table. I found mine a bit finicky to adjust on the Z axis, but I guess that's the price you pay (or don't). I've read about people having issues getting straight cold rolled steel lately but I did not have any problems. I've used mine for about 2 years of hobby use and they still work fine. My delrin acme nuts are wearing out before the bearings on the carriages do. You could also replace all the bearings fairly economically if they did wear out. I think one guy who was running production with them did that after a year or so.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html