I got the M12 inflator to replace my old Ryobi one, and it has been great for me, but I just have a Prius and Tacoma to take care of, and occasionally my daughter's Civic, so can't make a comment about any heavy duty use.
I have both the Fuel and non-Fuel 3/8 ratchet, but for some reason (I don't know why), I grab the non-fuel first.
I have both M18 impact wrenches and the M12 stubby. I always grab the M12 first unless it can't handle the job - so much lighter and easier to handle, and gets most things done (though as stated, I'm not a pro (my wife says so too...)).
Although I have the 15ga and 21 deg framing M18, I also have the M12 pin nailer, and it works better than any of my pneumatic pin nailers.
I also tend to use my M12 impact DRIVER more than my M18 (for the same reasons as the impact wrench).
I have the M18 drill, but not the M12, so can't compare. If I did have it, I would likely use that more often too (see impact wrench and driver reasons). I DO have the quirky "installation Driver" (2505-20) but I actually use my "regular" drill and/or driver more often. I got the installation driver more for the different heads in case it's needed. It's supposed to be a great tool though (other than the weird handle setup), and I've had no issues when I have used it.
I also have the M12 right angle die grinder, and it's true what people say about the lack of power/cutting out if too much pressure is applied, but I've only used it a few times, and mostly use it for Roloc stuff on hubs and brakes. I wouldn't buy it again. In fact when I decided to get a cutoff tool, I just went with the Ryobi because I read that the cutoff off tool has similar limitations.
The M12 palm nailer works great - better than any of my pneumatic palm nailers, but it's bulky as h*ll and heavy (compared to pneumatic).
Other M12 that I have but have not used enough (or not even used yet), so can't comment - right angle non-fuel drill, Fuel mini reciprocating saw, ProPex expander tool.