I am slow, but here's what I did to feel really confident, IFR, in a G1000 C172:
- King School Course
- 3 hours ground
- One VFR flight with instructor
- Two IFR flights with instructor
- a couple of local solos, including one in .2 of light IMC
- throughout, read and made notes in Max Trescott's excellent book on the G1000
After that, I felt ready to do a long (600 NM, round trip) IFR X-C to visit an old professor. That flight really solidified everything for me, and sealed the deal on my attitude that the G1000 system is highly capable and highly desirable. I've never flown anything "below" a complex single that had what the old Cessna 172s and 182s don't usually have: traffic, terrain, and XM weather. (And this one had a stormscope, too.)
There are so many nice elements to the system--all those safety items, the big display, etc. The only thing is, you gotta maintain your memory of the knobology, and you have to watch out for new kinds of gotchas. Make sure you know how the fuel levels are being displayed, don't let the extra "stuff" on the panel lead you into temptation, etc.