I have one, they offer superb picture quality. I have it running on a regular 17" CRT Monitor.
The only problem is they don't work on a standard UK xbox, you would need a US or modded UK machine (set to NTSC mode)
Setting to NTSC mode makes the hi-def modes 480p,750p and 1080i available. This adaptor will only accept these hi-def signals. Most games even UK ones are at least 480p.