Pros
As others have said, you pick your jobs based on where/when you want to work. The wages aren't bad for the work expected considering you're a fill-in for another company. Show up pay for cancelled/rescheduled jobs is a great gesture (though I can imagine it not being enough if you travel far for the job). Good opportunity to network with other merchandisers you work with. Unlike many other companies in this industry, they actually give you reasonable/realistic estimated hours for a job. You will go over or under sometimes but you're never expected to do a 4-5 hour job in only an hour. Extremely helpful/friendly staff when I've had to call in with an issue or question. Internet friendly. Aside from your timesheet you don't need to show up with a folder full of papers to follow/fill out (unless you're the team lead, I suppose). Everything else is done online/over the phone.
Cons
Need more transparency when choosing a job. I would rather see the specifics of the job before I apply rather than after I get accepted for the job. Again, as others have said, you could work consistently for a few weeks then not work again for months at a time though this is obviously more a matter of the clients not having available work than jobs being held back. While show up pay IS nice, sometimes I do wish the clients had a more responsibility to inform you (who can then inform us) when a job has been delayed/cancelled. Case in point: I showed up for a job I had applied for two weeks earlier, only to find out it was already done by the store employees BEFORE I even applied.