i've used yahoo several times. it's easy to use and setup. commish can set scoring rules, edit rosters when needed, etc.
my recommendation if you want fun, don't join a league with more than 12 teams. the talent is too thin and even getting players off waivers can be a pain in the butt as everyone is waiting to poach the next breakthrough guy.
10 team leagues with small benches are generally the best. the small benches discourage hoarding players, and you won't have trouble finding a #1 RB in a 10 team league. the #2 might be a bit trickier but that's what the waiver wire is for right?