This is a quintessential Caribbean beach. Perfect turquoise water, white sands, palm trees, and a quiet, relaxed feel. Unlike Maho Beach with its landing planes or Orient Beach with its crowds and activities, all that seems to go on at Mullet Bay is swimming and relaxing. There is one section of beach chairs on the south side of the beach; the rest of it is empty and it's easy to find your own secluded spot if you'd like. The south side does seem to have the best swimming, although we didn't go all the way up the north side, but the sand did get a little rockier (especially in the water) the further up we went.
Also, we found it interesting how different the people were depending on the beach. This may depend on the day and admittedly it is highly subjective, but when we were there, at least, Mullet Bay beach had a younger crowd than any of the other beaches we visited.
The waves when we were there were wildly inconsistent - none were unmanageably huge but a few of them did definitely reach as high as 12-15 feet before breaking. The people who were in the water at that point mainly were ducking under them. In between these, though, there'd be periods of 4-5 minutes of totally calm water. It was weird - I've never seen water act like that, but I like big waves so I thought it was cool. From what I've read here, on most days the water's pretty consistently calm, but it wasn't when we were there, which was one of St. Maarten's weird winter days when it suddenly gets stormy out of nowhere before becoming sunny again 5 minutes later.
If I were going to take another trip to St. Maarten, this is the first beach I'd visit and I'd probably stay there all day. This is not the beach you want to go to if you like a lot of amenities or activities, though.