In our dealings with them, the staff at Rainbow Reid (usually David Silver) were polite and helpful. Paperwork was completed efficiently, and they were happy to negotiate a break clause in the contract with the landlord. Towards the end of our tenancy viewings were always conducted with the required notice.
My only gripe with them is the check-out company they use. Our check-out report was a complete hatchet job, included accusations of us smoking in the flat, despite the flat being non-smoking, and both myself and partner being non-smokers. I had to write a 25 point rebuttal to the check-out, in response to which the company made it quite clear that their word was final, despite how petty/plain wrong some of their observations were. Luckily our landlord was really understanding and decided not to use the report to make any deductions from our deposit.
I would recommend Rainbow Reid as an agent, but be very wary of the check-out people.