This subject always seems to polarise opinions, I've been on some forums where the masses say flushing an engine is the worst thing you can possibly do, and will certainly definately kill your engine and all those around you at the time.
I've done a few flushes in the past using Wynns stuff, and not had any problems.
I think the general worry is that you'll loosen gunk inside the engine that will move around and block things up, causing issues.
A specialist I used to use would flush an engine a few times using an oil/diesel mix before putting in the proper oil. Claimed it left the engine sparkling inside.