Pros
Health benefits for part-time work. Company has really good ideals even if the daily reality falls short sometimes. They aspire to treat their employees well, even if they never achieve the utopian employer-employee they strive for. Perfect your social skills. Free junk food. Once you work at one Starbucks, you can get a part-time job in any town. Flirt with girls or guys. Relatively easy job. Young, fun, if sometimes immature coworkers. Perfect job for extroverts.
Cons
You may get fat off the drinks and food. It's a retail job so expect retail pay and retail respect. You feel a little nasty and dirty at the end of your shift. On the retail side, retail career advancement isn't so hot. After being a barista, you can move up to shift lead, assistant store manager, then manager. Each move is hard since there are too many who want the job and are capable of doing the job. People wait years to get an Assistant Store Manager job paying a little over 30k or Manager job paying a little over 40k-50 depending where you live. In that time, you could have just got an office job that pays more for less work. However, once you make it to manager, then career opportunities in the company really open up.