Not much of a weight penalty on HUMS as every 139 built has all the wiring and all the gearbox accelerometers fitted as standard. Getting the complete kit adds on the Computer under the baggage bay floor, the tracker camera in front of the co-pilot, the RTB accelerometers and the cockpit display unit.
Probably the best track and balance computer I have seen in the last 20 years. The track and balance functionality has been flawless on the 139's I've worked on.
I've seen a few problems with accelerometer wiring giving erroneous readings on a gearbox, but overall the vibration monitoring works very well. Easiest thing to do is have a support contract with Smiths/GE and/or Agusta so that they can monitor your machines for you and give you some advice on what to look for on the machine.
Overall I personally am very very happy with the HUMS as fitted to the 139.
The one thing on my wish list for it is some kind of function so that we can operate the HUMS in the hangar when we are tuning the underfloor vibration absorbers. At the moment we have to use an 8500 for that.