We operated a 26yr old A109 until about 4 months ago.
It was a maintenance nightmare, every trip something failed. Spares were no huge problem to obtain, there are still a fair old selection of parts available for aging 109s.
We do about 35-40hrs a month and mid last year we managed 2 days a week servicability. In one month we replaced a tail rotor gearbox due to chips, a torque matching linear actuator, a transponder, a VHF radio and had an engine chip light. This was beginning to be typical for a month.
The DME and RADALT could never be persuaded to work properly, if at all from purchase, depsite being looked by at least 3 seperate avionic engineering companies (no criticism of them, just a pig of a job).
We swapped it for a 109E which is a lot less hassle, but it still has a few issues, primarily related to what i would call 'engineering induced faults'.
There a few decent aging 109s around, but anyone considering buying one wants to have a serious look at it first.
I love the 109 its a great machine to fly, and the new ones can carry fuel and a few people which is an improvement over the old ones were it was an either or option.
GS