I use this all the time for my workouts at the gym.  Whether I go in street clothes and carry my workout gear and shave kit, flip flops, towel, etc, or if I go in workout clothes and haul street clothes, it has enough capacity for everything.  What I really like are that there are two large side pockets that are waterproof so if you have soaked, nasty workout clothes or wet shoes or such, you can put them inside these pockets and keep other items dry in the main compartment.  It's pretty lightweight and at the same time holds it's shape well.  Would make a good weekend travel bag as well.

Plenty of small pockets for cell phones, iPods, wallets, etc as well - great gym bag and at $50 for some colors, a great value.