Do it part-time or as a fill in until you can get a better job.
Bike shops = low pay and long hours in the peak season. Saturdays and evening hours are usually mandatory. Forget about any time off as most shops are understaffed. If you think you're going to be a mechanic that gets to hide in the back and work on bikes all day, think again. You'll be answering the phone and manning the sales floor as well and if the shop does fitness equipment, expect to do service calls, deliveries, and maintenance on that as well.
Customers are another kettle of fish and I'll let someone else chime in on that - as we get alllll kinds. Everything from the nicest people in the world that just want to buy a new high end bike to pain in the @sses that are angry because it's going to cost $200.00 to fix their Wal-Goose that their kid trashed in a few weeks - and everything in between. Maybe we need a "Customers suck" sticky thread...
The job can be quite enjoyable in spite of all this. I do it because I like the people I work for, I like working on bikes, and most of the customers are good people. The discounts on bike goodies is an added bonus. It's a fun job in small doses but when it turns into 45-50 hours a week, the fun factor disappears pretty quick.