We have been renting a flat with QB Management for a year in the past. The only thing I would say wasn't much impressing (it may not be the case with other properties) - agent who was doing a viewing for us didn't know much about the property - i.e. furnishing, heating, water tank etc - most of the answers were "I'm not sure, will need to check". In my understanding, agents should be quiet knowledgeable in this matter or at least have some notes to be able to answer simple questions like if there is a gas or electric heating for instance or which pieces of furniture comes as a part of flat furnishing.
But all other aspects were very satisfactory. Signing the contract, checking references etc. - service was very fast and all staff very. Also, communicating our requests to the landlord was really sufficient. We had couple of issues during the tenancy with the plumbing and heating and these have been promptly and timely addressed. Checking out from the property was "painless".
I would definitely recommend them!