B52 is a nice place. Too expensive for the backpackers (who frequently can take over a place), but not too posh -so still having the feeling of being in Thailand, which feeling the staff gives you as well since they are very nice and like to joke around. The pool is nice, but lacks places in the shade. The beach, by far not the most beautiful on the island is rocky and the water can be very far out (although a lot depends in which time of year you visit -tides in the Thai winter are favourable here). Foord is great and they have a nice pool-bar with great cocktails. Not the cheapest. Bungalows are basic, but provide all you would need. Thong Sala is very close by, so nice if you like local food and a street-market vibe. There are a lot of cheaper places nearby, but none with a pool as nice as here. This is probably what you pay for. So if you want to be sure you are accomodated well within nice grounds, and don't mind paying a little bit more, this is a great place. Though they should expand places in the shade.
Koh Phangnan itself is an islang I have visited a lot in the 80's and early 90's. Though I think it is still a nice place, it can all be a bit confusing, because this Full Moon Party seems to dominate (at least this part of) the island. Either all is very crowded or either feels a bit deserted. When outside of season a lot of places are closed, or have no vibe, during fool moon often lacking place to even be seated. I think when you choose B52 either way you would be happy. Again staff is very kind and good to have more than a few laughs with (which is why I am in love with Thailand). Besides the food the food that is.