Gap Inc's strikeback at Lululemon works.
The best comparison to Athleta is Lululemon, except that Athleta is just more accommodating. The most radical difference is Athleta's super generous return policy. I would say that Athleta is also a little more casual wear oriented than Lululemon. Like Lululemon, they offer free pant alterations, weekend free exercise classes, and always a lot of new styles of tops and leggings. The store's inventory is much smaller than online, but if you use the computer inside the store, you can order whatever you like with free ship. The back of the store does offer a couch for tired shoppers, as well as a lot of free class cards at local gyms.
Athleta is superior to Lululemon for me. Sometimes you just gotta return stuff.