The ADuC series are basically ADCs that come with '51s or ARMs built in, and not the other way round. If the ADCs weren't good, they'd have a hard time selling the devices at their current price point.

That aside, I have used the ADuC845 in one project and don't have that many complaints about it, except perhaps that it goes into idle mode while writing to the program memory eeprom, but I guess that can't be helped. We were using the device to do some fairly high-precision measurements (temperatures within a 40 Kelvin range and a precision of well below 1/10 of a degree).

It also generates 500,000 baud fairly accurately.