I've been to mellow mushroom on High Street a few times. Solid pizza, average service, a little overpriced. I love, love, love pizza and have to say there isn't anything super special about Mellow Mushroom, but it definitely satisfies the need for a good pizza with fresh ingredients and great flavor.
The only reason I don't come here regularly is the service. I've sat at the bar and the dining room with my family. The staff seems more interested in each other and their social happenings than working, and have had several slow encounters. I wouldn't mind paying the premium they charge for pizza if they could hit that x-factor for service.