Lake is one of the tourist spots on the way to Nathu La pass, and just like everything around it, is popular because of the breathtaking location it is situated in. Lake per se, is large but nothing to talk about. The water is clean but not clear. Boating isn't permitted.
Cable Car at lake (360/- charge, 3yr older) is worth its price in beauty and experience. The ride is about 5 min each way, and you can spend half an hour at the top taking pictures and soaking natural beauty. You can have tea/snacks at top (40/- tea, on the expensive side). The cafe at the bottom has a limited vegetarian menu with not so exorbitant prices.
Pictures with the backdrop of the lake give you the most poster worthy pictures in our whole East Sikkim trip.
There is an option for Yak rides around the lake but we found prices exorbitant. About 1200/- for a 10min trip seems expensive even from tourist place standards.