I've stayed in Lean -to's a couple of times.
Once on the Appalachian Trail in Maine and a few times hiking Isle Royale which I did 3 times.

The Isle Royale shelters were on small lakes and my group of 4 had them all to ourselves.

The one time we stayed in Maine another group showed up and they wanted to stay in the shelter too.

It was 8 people plus their dog which made for some cramped quarters plus some ill feelings before we all left the next day. I ended up setting my tent up on the edge of the site to have some privacy.

If you can get one for yourself of just your group they are indeed a nice respite from setting a tent/tarp camp.