We had 20k of work done, with a friend of ours as project manager. This involved coordinating several different trades and though it was no quicker than using a single builder, it was definitely cheaper (well, we ended up paying the same, but for a higher-spec job).
He then managed another job for us, where he produced a detailed spec for a builder who produced an equally detailed quote, including prices for anything he was subcontracting, and also sorted out the sparky. We ended up with some unexpected extras, but that would have been the case whoever we'd used (new boiler), as well as some things being cheaper.
BUT
a) our friend is a project manager: that's his job. He has an excellent local reputation, works with English Heritage etc etc and knows a lot of really good tradesmen.
b) there were a few jobs that we agreed the builder wouldn't be responsible for and I'd find someone to do. This worked out well for us, and it's easier to have that kind of flexibility. If I hadn't been able to find someone, our friend would have done.
If your friend has a good reputation, as good as the main contractor, I don't really see what you have to lose.