Pros
Easy enough to register with them, I've had good pay every time I've worked via them (for someone under 25 anyway) but I imagine that just depends on the company who's hiring them. Both of the places I've worked at have also offered opportunities for permanent positions, albeit one only in theory (though this contract did get extended several times).
Cons
Most of the downsides have been issues within specific companies but I've never had any personal problems with Adecco, although I've heard anecdotes about delays with pay etc from people I was working with at a factory which sadly give me the impression that people doing admin jobs are treated with more respect than people in factory jobs as when I had pay issues in my admin job they were resolved immediately.