After 6 months you can get tuition reimbursement from best buy, goto school get a degree(I HIGHLY recommend taking advantage of this). You're experiences at geeksquad/bestbuy will vary quite a bit(different locations can have very different management), if you're able to put up with a lot of BS you will do just fine though.
As far as competence and management goes from what I understand that varies dramatically throughout the company. As a senior, and please don't use the full job title lol, you will probably end up dealing with customer issues more than fixing PCs or at least that's what that position generally entails.
Parts of the job can be very interesting, you see a large variety of people everyday, some days you'll lose faith in humanity all together and on some days you might meet cool people to.
People are apparently able to use this on their resume and get higher paying jobs(i know many people who have gotten IT jobs after working there a while).
Oh yeah one thing you'll run into a lot of people that feel entitled to something. example: "I bought this computer at compusa(person literally had the receipt from that store and everything) three years ago and it won't turn on I want you to replace it!" "no, we're not compusa and they probably won't replace it either" "BUT IM A LOYAL CUSTOMER I JUST BOUGHT A TV 3 MONTHS AGO!!!" lol not joking about that either.
In the end you'll always have a story to tell....