As an ex member of staff, I consider myself a good candidate to provide a review. Before you post replies saying that I am biased, I would advise that I have had no contact since leaving their employment and so have no reason to compromise my integrity.
I worked for studentDigz during 2004/05 and there was never any question of keeping a tenants bond. The company recognised that as a letting agency, they need to provide a service to both the landlords and tenants and in any company this can prove difficult.
Regarding the cleanliness of a property - most tenants are young students who are living away from mum and dad for the first time, their idea of clean is very different from adults that have spent many years in the real world.
A rental agreement should not be a stressful contract. As long as you ensure you honour the clauses in your contract, pay your rent on time and keep written and photographic evidence, there is no reason for your bond to be kept.
Emergency numbers were always given out when I worked there and staff would always try to help if a student had a problem - whether it was related to their tenancy or not!
Now that I am a landlady myself, I would be happy to use studentDigz to rent out my own properties.