They are fun and pretty much same difficulty on most fights, can be harder on other fights.
Only problem I have is the game isn't optimized well for 16 people and if you don't have a great machine, you'll take a massive hit in FPS going to 16man. I drop from 30-50 fps in 8mans to 15 or less in 16mans.
Hmm, the frame rate hit is one of my main concerns. That's a shame.