fast way to get around bangkok. during our stay, we used bts for almost all of our trips around the city. it is clean, comfortable and AC works great.
Avoid rush hour though-- you surely don't want your face stuck in someone's armpit.
Ticket machines accept coins only so be ready to have coins or have your bills replaced with coins at the ticket booth.
I'd recommend using BTS (aside from walking around). its cheap, very convenient, you get to see and feel Bangkok more plus you get to avoid traffic and cab/tuktuk drivers who will try to rip you off.