Well guys... Here's My 2 cents!!
I've been a professional handyman for about 35 years, and a bit of a "tool hound".
The Multi-Master is one of those tools that has a major place in my truck. It is really NOT an everyday tool, but what it's designed for it does very well.
Re grouting?? Nothing else in my years makes it easer than the M-M and the carbide blade to remove grout. Makes FAST work of removing old grout and easy to control. And it makes quick cutouts in ceramic, not fire hardened, tile. Haven't tried diamond blade yet??? Bit expensive for the number of times that has come up yet.
Removing a damaged piece of baseboard? No other tool I know can do it any way near as fast and easy, without damaging either wall or floor.
Cutting into a drywall wall? Very little dust, very narrow cut (easy to use cutout to re patch), and relatively quick.
I had a job for a customer that wanted the "raised panels" in the top 2 sections of his ceder garage door removed and glass (fancy) installed. there is NO other tool that I can think of that could remove the inside part of the rabbit that holds the panel so the wood panel could be removed without damaging the door. Took me about 4 hours to remove 16 panels.
Inserted the glass, made some new quarter round, (router table) and viola, the hard part was done.
The thing that makes this tool so unique is the control that you have on it. There is no shaking, no kickback, it cuts where you put it, and can cut flush to something you DON"T want to cut!
This tool has been out for many years, but it's just now that so many companies are making copies of it. I have tried the Dremel model, and took it back after an hour. Not even CLOSE to the quality, Didn't cut for beans, and vibrated like mad in my hand (like using a palm sander for hours.) I've heard that the Rockwell version is fairly decent. And just saw that Bosch has one, pretty decent company. And today I found that Home Depot sells the Bosch blades, (flush, half-round, wood/metal blade $25.00). Haven't tried it yet, but Fein and Bosh are both German companies and hopefully have the same quality of blade. The Fein blades are of very high quality and DO last. We'll see about the Bosch... Don't even consider the blades from Harbor freight, they didn't cut cr*p, but for $5 I thought I'd give it a shot.
As far as the problem of "heating up", whoever it was, I've never had a problem at all with that, even when cutting all those door panels. Th secret, which shouldn't be, is to let the tool do the cutting, and do not force it. A light touch means better cutting, more control, and longer blade life.
Again.. not an everyday tool , but will do jobs that no other tool can do.