I believe there are multiple MU lounges, including one near gate 17 that opens at 4:30 am. If this is the one I'm thinking of it is relatively nice -- two floors, lower floor has hot food and tables and can be pretty congested. Upstairs (escalator) has more comfortable seating, much less crowded and sandwiches, fruit,etc. but no hot food. Staff did a pretty good job of keeping chairs/tables upstairs cleared and clean, at least while I was there. Toilets are clean and serviced frequently as near as I could tell.
I was there from opening at about 4:30 am through about 10 am and on the return from about 5 pm through 8 pm.
Also several priority pass lounges according to the directory but I didn't wander around as I was comfortable in the MU lounge.