If you have a lot of chunks floating around in your tranny, you've got bigger problems than a fluid change of ANY type is going to solve. A yearly double drain & fill has worked fine for me (130K+ miles) and is the recommended procedure in several manuals. I've heard to many horror stories of catastrophic failure after a pressure flush to have confidence that the shop I use is going to do a power flush correctly. The most important thing imo is to get clean fresh fluid in the tranny before it degrades due to heat. I agree a flush is more thorough, but see it as riskier overall because it can cause damage if done improperly.
