Pros
I worked in a flagship store in New York City. The Genius Bar is fun gig if you don't mind dealing with people when they're freaking about a broken ipod (I didn't mind at all). I met lots of great people: fellow employees, celebrities, academics, artists, musicians, models, etc. The benefits were stellar: 401k, full medical/dental/vision, two weeks paid, discounted stock purchase program, discounts on merchandise.
Cons
As a Mac Genius, you are fairly replaceable. When I started at Apple, being a Mac Genius was really special. But by the time I left, after the company had expanded rapidly and opened dozens of new stores, the talent level and expectations had dropped considerably. Therefore, it was not uncommon to get fired for minor offenses and starting salary had declined precipitously for MG's by the time I left (as much as $10/hr less). Higher retail managers were often drafted from places like Gap/AF/B&N, etc so they had little technical skill and product knowledge. This diminished the culture somewhat for die-hard Mac heads like me and other MG's and salespeople. Management was overly self-congratulatory about their sales figures: anyone can look like a brilliant sales manager when the product is iPhones, Macbooks, and iPods--this stuff sells itself. Don't forget that, ladies and gents. As a Mac Genius, there is little opportunity to move up. "Lead Mac Genius" is a supervisory position that very few attain and it carries little if any bump in pay. Like many companies, Apple has a bad habit of hiring management from outside the company (for some reason, someone from someplace else always seems more talented than the good soldiers you have in your own back yard). I still know extremely talented leaders who are MG's who have been passed over time and again for management positions.