I haven't been to the M Resort buffet in years (more than 5), I didn't think the absolute quality was as good as Wynn/Bellagio, but it was definitely a better value.

As for Cosmo, also probably has been > 5 years. I thought it was pretty good, if a little gimmicky.

Self-service beverages is a bit tacky. Ideally, your glass would be topped up each time you came back with a new plate of food, if the service was any good. Now you have to go carry your food back, put it down, and then go get a drink? Or juggle a plate and a glass? Hmm.