Affiliate Marketing - Advice needed

We are an Office Supplies company who are venturing into the world of affiliate marketing as a vehicle to increase traffic to our site, www.officeallsorts.co.uk .

We have done a great deal of research and spoken to a number of the big players as well as some of the smaller up and coming players and those based abroad.

They're not cheap and to be honest, they all promise guaranteed returns,.... too good to be true?

I'd really appreciate hearing people's views on affiliate schemes and the experiences they have had, good or bad.
Did the result exceed the promises?
Any recommendations of good affiliate networks?
Should we think about engaging an affiliate management company to manage it on our behalf?

As I said earlier, it's not cheap so really can't afford to experiment to a great extent so any advice would be greatly appreciated.

Kind regards

Tim