Pros
I can't think of any reasons to work for Robert Half that are over or above the same reasons to work for another company. They've been around since the forties, so they've got staying power going for them, so that's good. They also have world-wide offices, so once you get your foot in the door, there could be the possibility of moving to another office if you like - there is a lot of turnover and so lots of internal postings for open positions. They publish a salary guide every year, so there's no reason for their employees not to know the going rate for a particular position, so you know they'll be paying the industry average, more or less! They give prizes for longevity.
Cons
It's a huge corporation, with all that implies. It takes FOREVER to get things done sometimes. The business model is sell-sell-sell and for the salespeople on the floor, it's very stressful. If you're not a type "A" big time seller and marketer, you probably won't last for very long. The administrative staff are not treated the best, maybe because they're not making money for the company. They're not outright mistreated, but can be micro-managed and taken advantage of. Corporate sends out memos detailing how well the company is doing and how much money it is making, but nickle and dime the offices until you want to tell them to shove it!