Room: I stayed in a 6 dorm bed, which had its own bathroom and shower (clean enough, but definitely worn--doors were kinda grimy). The shower had hot water, which was a pleasant surprise. The beds were comfy enough, and the AC made the heat bearable--when it worked (only turned on at night). On Friday night, the AC didn't work, so I was miserably hot and sweaty all night. If the windows are closed, it's not hard to sleep well at night. Bad wifi in the rooms, decent in the bar/lounge areas.
Environment: Selina is a great location for parties, it's usually where the Filthy Friday crowd ends up at night. The hostel itself has a nice bar area right on the water, so it's perfect for swimming. There's plenty of lounge space on the ground and second floors. The main drawback IMO is that there's loud music blaring all day and night, so it's not the best place to really soak in the beauty of Bocas or chill by the water.
Service: takes forever to check in/check out/get anything done, but the staff is friendly. Food is decent but you might as well get better eats outside the hostel. Convenient transportation to other cities; took the Bocas Transfer to San Jose and didn't have any problems.
Overall: if you're staying in Bocas for more than a couple days and want more than loud partying, spend your Friday/Sat night here and maybe go elsewhere for a more chill environment in a quieter location.