I'll mirror the first two responses- hobby pilot with a strong likelihood of staying behind a particular panel should train using that system. Professional track pilots who need to be able to adapt quickly to a variety of systems should get all their tickets on steam, and I would recommend against using the G1000 for primary instrument training for anyone who does not plan to use the Garmin exclusively afterwards. After CFI it doesn't really matter what you use because glass is easy enough to add and going forward is much easier than going backward.
GA glass systems were intended to consolidate the functions of previously isolated panel instruments. The general trend throughout history has been to more panel organization. All-glass was a natural consequence of the existing trend but it skipped a beat in the process. There was a point in the early 1990s when a few glass instruments were finding their way into GA cockpits but a wholesale switch to glass panels did not occur until Garmin lowered the cost on glass enough to permit volume sales. Thanks to Garmin for doing that and their system is great, but the overnight swap left a hole in primary training that we are still trying to fill.
My personal feeling that basic trainers for the private and instrument ticket should have little or no glass in them at all. When I taught on the G1000 system, my students were swamped with needless information and I often had to spend the first 20 hours getting them used to the panel when they could have been concentrating on basic flying skills. It bothers me that primary training is so often done on the G1000 system because it steals away the chance to learn a far more flexible set of skills and engenders dependency on the Garmin automation.
In addition, I would be remiss not to mention the G1000 is the worst possible system to use for IFR currency training, because there is so little mental work to be done using it. Situational awareness gets swapped for a moving map display, eye hand motor skills go completely on the shelf, and the ability to use older airplanes goes completely out of range. Garmin now has an autopilot (GF700) that even accepts vertical programming which removes even that problem from the table. Anyone using the G1000 panel for IFR currency should be issued a friendly warning they had better stick to the Garmin system because if they jump back to into steam without a refresher they will likely make an untimely rendezvous with a hillside.