Well my hd box pro is now here and working well. It outputs a wide range of resolutions including the 1440x900 of the small widescreen vga monitor I'm currently using. I plan to replace this (it was a kind donation from a mate) with a 4:3 monitor eventually.
Initially I was using it with the xbox still outputting interlaced video, and it worked well although there were a few glitches in some arcade games. This is caused by the hd box pro struggling to process the picture because it's not getting a full frame every time.
The biggest improvement I've noticed over using my LCD TV is reduction in lag. It's quite amazing what a difference there is, my TV must have been introducing a fair old delay from all the deinterlacing and scaling.
Games are starting to look really good now. There is still some scaling going on of course because the xbox outputs 480 or 720 which the hd box pro scales up to the native lcd monitor resolution, but the results are still very good.
By the way I had a hell of a time getting the xbox into prog scan mode. I had to add eeprom_off.bin and video.bin to a specific folder to get the virtual eeprom off so Enigmah could make the changes permanent. A lot of info I read on the web was wrong because there are different versions of unleash-x. Hopefully now I can remove these files but I'm not sure the virtual eeprom will have been updated with the change to the real one.