Leon's Library Blog has an interesting piece on the Amateurisation of Public Libraries. CILIP, the official body for librarians, has come firmly against community managed libraries, particularly because of job substitution of paid staff. That is, I would think, a fairly obvious f0or any kind of trade union to do.
Brent is actually quite unusual in this respect. Most authorities where there have been cuts to the libraries budget (which by now is pretty much all of them) have chosen to offer a "Big Society" route. When Brent was mulling its proposals, this was part of the zeitgeist created by the newly elected coalition government. Brent considered that option, but rejected it because it by no means clear that that would save money. Many other authorities _ Surrey, Gloucestershire, the Isle of Wight, Lincolnshire and many others _ have embraced it. People opposed to library cuts in those areas have therefore tended to oppose volunteer libraries as inevitably inadequate.
In Brent, after the handing over of public assets was rejected, founding volunteer libraries became the main focus of the campaign including the legal challenge where the Judge confirmed the Council's position. One group, Barham, appear to have succeeded in running a library cum bookshop on Wembley High Road on a stand alone basis. Two more, Cricklewood and Kensal Rise, have plans to run some sort of facilities on the former library sites. Again, these are independent of Council control. The last group still hoping to run some sort of library (although I would suggest calling it a "library" is likely to mislead) is the Preston group. This is still aiming to get substantial Council support, a level of support that I think is highly unlikely to be granted.
The great danger in handing over a library to volunteers is that is continues to rely on the Council for funding and various other kinds of support. This can become a substantial drain on library services that are already under pressure. I am sure that if Brent had gone down the route of supporting the various voluntary groups that wanted to run their own libraries, officers would not have been able to deliver the level of success that the Libraries Transformation Project has attained.
A somewhat off-topic comment below. As I have explained before, Brent Council did not "give away" a building. To do so would have been unlawful. It was legally obliged to hand the former Kensal Rise Library building to All Souls College. The full details on this subject can be found here.