We've had HUMS fitted for some time now. All in all, false warnings have been reduced to a very low level, and real alerts are now few and far between. We do get some problems with the fiddly connectors but all-in-all, I think that it is doing its job.
I agree that in-flight warnings are not a good idea using this system, in fact, they were originally designed-in for high eng vibes, but inhibited shortly afterwards.
The system does allow a complex series of measurements to be carried out during flight, and analysis carried out on the ground. As the system gets older the alert thresholds are 'polished up' and you end up with a system with years of experience available as an off the shelf product, with each aircraft fitted with the same level of monitoring.
Some of my workmates have not had training on this system (which is a shame) as they would find that it is a great diagnostic tool which supplements existing maintenance practices. I've heard that training is mandatory in the UK. The days of rotor track and balance (RTB) defects taking multiple test flights are over, with one or two shot adjustments giving engineers more crew room time. RTB adjustments are allowed without airtests in many cases.
The addition of the HOMP (Helicopter Operational Monitoring Programme) using CQAR data has been found to give engineers alerts to dormant defects automatically. I guess it tightens up pilot flying practices as well.
I was a dubious user of the system in the early days, but as you can see, now I've seen the light. If I was a helicopter pilot, I would be wondering why I've not got it fitted on my aircraft. If I was a fleet manager, I would be doing some serious sums as I'm sure that HUMS saves maintenance time and, of course, money.