Noisy,
There are varyious levels of checks which can be carried out and the length of time taken can vary a lot. A standard police check takes next to no time at all, whilst an enhanced crb check can take much longer.
I keep myself very busy as a volunteer on a cadet squadron and also a vgs and although I already had an enhanced check through the vgs, when I joined the cadet squadron another was required. I also have an enhanced check for my part time job working as a taxi driver, and all of these checks are required to be renewed periodically and independantly. This is because a check is specific to a job/company and is printed on the disclosure. (Just glad I don't need one for the day job too!)
I would personally much prefer these thorough checks (even if they take a while) than for people to "slip through the net". And as has already been stated with new staff there is nothing to stop them instructing or joining in so long as they are not left alone/unsupervised with cadets.
Two's in,
To answer your question, I've visited a number of vgs's as a relief instructor over the last few years and experienced very few problems. My own experience as a cadet and as adult staff has been a very good one and I have yet to meet a cadet who hasn't gained from the ACO as a whole. I can't comment on the experiences that have been raised by a couple of posters on here, but can tell you that I have personally never encountered anything similar myself.
Safe flying to all.
Chinchilla.