Some are, some aren't; the main agency I use is a one man operation who seems ok - he's provided me with a fair amount of work in the past and he's happy to negotiate different options eg when I started I was still at college which didn't seem to be a problem, while other agencies wouldn't even consider me for employment.
The most recent agency I've worked for are slightly larger and it shows. When you call them they haven't got a clue who you are, don't seem to know what each other are doing and really don't care about their temps - I had to have a few weeks off due to an injury at work and they basically spoke to me as if I was just a nuisance because they would "have to" replace me. I spoke to someone working directly for the client and they seemed fine with it and happy for me to go back - it seemed the agency just wanted to have someone on the job continuously so they get their commission.
National agencies like Hays, Adecco, Reed etc don't seem to be worth bothering with - they seem happy for me to fill in a massive form, then they always tell me they haven't got any jobs.
Bear in mind there isn't a lot of work around at the moment (there may be a rush on in the last week of the school holidays) so firms seem to prefer to have their guys working weekends rather than using agency staff.
