I was a landlady for 5 years when we rented out a flat. I didn't use an agent, preferring to vet the tenants myself, and I never once chose badly. When we sold the flat it was left cleaner than even my high standards for a flat on sale would have achieved! The place shone. We didn't have to pay CGT either, something to do with the fact that the flat was our primary home for a while, plus a whole host of other factors, calculated for us by our accountant.
So I think that if you suss out the rental market in your area, buy a rentable flat, make sure you get tenants that you know will be good (check references, ask to see pay slips etc, it's not that hard - plus give them a good once over yourself, interview them) - then it can be a very positive experience. And we made a good profit out of the flat when we did sell it, even with taking into account council levy, insurance, gas certificates, maintenance and repairs. In fact when I find another property to invest in, I'll do it again.
On the other hand, I think many agents just take their commission (some up front for a year) and after that don't give a toss.