Pros
Super simple job literally anyone at all could do, lots of opportunities to pick up hours if you need more work/money. Because it's a crap-paying job there's a lot of teenage/inexperienced employees, who don't necessarily have the most forethought, so they don't think to tell management they need time off for family things, for school stuff and so on, so it's easy for the cream to rise to the top, if you will. You look especially good if you're willing to jump on those shifts that are always being left uncovered, so when keyholder and assistant positions are open you're first in line.
Cons
On call shifts can be a pain. Half the time, the people on-call wouldn't call (as per the policy, you call one or two hours before your shift) to find out if they were working and then couldn't be reached. If you're responsible and motivated, the upside is you can pick up a lot of those shifts the others didn't want. The downside is that you never really know if you work and it's not uncommon to call and be told "right now, we don't think we need you," only to get a call two hours later and be told someone didn't bother to show up (again) so they do in fact need you. It is a chaotic environment. I came from a store whose customer service was by it's nature very personal, very involved and required your attention. This is not that store (good or bad depending on the person.) It's easy to get caught up in how much needs to be done, the bigger picture, and feel like nothing you're doing is making a dent. As a new cashier, it's impossible to really learn the store. You're tethered to the register when people are there, so you don't learn where everything is, and so many people wait until they're checking out to ask for a specific product because they didn't see the only other person on shift on the sales floor. And you can't take them over to show them where it is because you don't know but even if you did you're the only person on register and you can't leave.