It's open 24/7 (but normal brekkie/evening apps are same time frame as non- M Lounges, aka Sun night through Fri morning), and like a lot of other normal lounges is closed on the weekend for evening apps on Fri-Sat nights. Some of them are near/attached to restaurants w/ hopes you'll buy food/drink via the M Lounge from them, thus increasing Marriott revenue (probably the bigger reason they were created). There are reports of some M Lounges being nice & others being 'eh'. Some of the items M Lounge touts, regular lounges have already.

M Lounge is a US concept, not int'l.

For those who are posting properties that have M Lounges, feel free to create a wiki

Cheers.