That is a good experience.  Are the M28 tools NiMH? 
The sad truth is that tho I love the M12 tools, how long will it be before newer technology will essentially obsolete them?  Or at least seduce us into moving to a newer system?
I have many 120VAC corded tools that are truly lifetime tools.  Some from the 70s.
		
		
	 
Yes, we also still have corded Milwaukees from the 70's, too. Nice old tools; they hardly ever get much use now that the cordless are so convenient. A couple of corded Craftsman tools do get a lot of use. Nobody else has yet made such a clever scrolling variable speed jig saw or really good sheet metal nibbler as those old 1970's Craftsman. 
All the M28 and V28 Milwaukee batteries are Lithium ion type. They may have been the first of Milwaukee's lithium ion line. Originally the line was V28, and them became M28. As far as I can tell, there is no difference. Same battery & same everything. 
They have seriously more torque than the M18 & double the runtime. You can still find some of the M28 tools new with a google search, but be aware they are large and  heavy in spite of most having magnesium alloy cases. 
None have ever needed any parts. Even the M28 batteries still work. 
The standard M28 hammer drill is still my favorite go to drill. Not too large and has full torque at a very controllable low speed - which makes it especially good for drilling stainless or thick carbon steel. Has a reliable hand tightened carbide faced chuck that doesn't bugger bits.
The M28 circular saw is a real oddball saw. It is "left handed" compared with other circular saws. I rarely use it for that reason. 
The big right angle drill is a monster. It has a big keyed chuck and when equipped with a "winch bit" adapter that drill was popular for use to power sail boat winches and anchor winches. I can't imagine what use that size tool would have for drilling holes. It takes two hands just to lift one. 
Nice for powering big winches though....
The M28 impact driver weighs an astonishing 20 lbs 
without the battery. That weight means  it doesn't kick back much and will loosen the lug nuts on most tractors....not on the JD310 rears though. Those need an air impact. 
More recently, we also have a little M18 drill and M18 work light. Both get a lot of use when I don't want to lug an M28 monster. And I'm looking forward to getting some M12 tools. 
rScotty