You'll get different opinions on all guns.
My take
The dm9 is the fith year of incremental improvements since dye took on the dm platform with the dm4, it is regarded as one of the best revisions (pb nation have a thread on the dm evolution with a table showing improvements etc).
The dm range at that age and newer have self cleaning eyes (up to a point, have fantastic ergonomics, operate at low pressures so are gentle on paint and are a very stable shooting platform.
Maintenance is very simple. Pull bolt, lube, put back in and the owners guides are up there with planet eclipse.
Spares are easily available as most of the parts are common, for example o rings, eye pipes, solenoid, boards and so on. Also you've got dye techs easily contactable if you get stuck Ash being a great example of one.
I was supported by dye in 2008, and have several friends who have or do work for dye. That said after returning to paintball after 7 years with no sponsorship ties etc, I've still settled again on a dm platform.