Pros
The benefits by far are the best reason to work for Starbucks: health and life insurance, 401K, stock option, free drinks, weekly markouts, and other perks. And these are all available to part-timers. Depending on the store you are at, you work with really good people, and it can be fun. It's easy to transfer stores if you are unhappy with yours. Making drinks can be fun.
Cons
Customers are by far the worst part about the job because they can be extremely rude addicts, but Starbucks policy turns you into their doormat. Don't try to move up in the company because they make it easy for some and extremely hard for others. This is not based on any professional standards by any means. Unless you plan on moving up to corporate, moving up at all is not worth it. Stay a barista. Do your 20 hours. Just enough for benefits. Have something else to do.