I've used the 8515 a little and have had little problems with it. It sometimes does not whant to enter the programing mode, but usually does after 10 tries or so. Some times the flash would not erase. But then again the programing circuitry was home made.
It took a while to figure out what was wrong with the multiplication. The chip does not multiple.

Thats why I plan to go to the Atmega. Whats wrong with ATMEGA?