These have been around for years...
And have gone off-patent.
What Fein had charged pretty for simply collapsed in price.
Harbor Freight is pitching an Asian import for peanuts.
The action -- for the manufacturers -- is in the blades.
Even Fein's don't last.
Their metal cutting ability does not extend to drywall screws, BTW. Even a single such attempt will ruin a $15 blade.
They are ideal for old work. You'll see them slipping into the frame of This Old House, Ask This Old House, and in the tool kits of every DIY Network host -- if you watch long enough.
For me, they are the only game in town for plunge cutting old work lumber.
My first usage, all those years ago, had the GC dropping his jaw. He'd hoped to have buried me with some plywood sheathing. No dice.
Multi-masters even change my build sequence. I use fewer 3/0 rings now - going square instead.
For new buyers, the issue is blades expense. You'll have to do your own homework: everything is changing with the end of Fein's patent wall.
I don't work in old plaster. But, if I did, I'd never use a vibration tool. >>> Crack-city!
You are much better off using the ancient classic: the beater. This is well exampled in one episode of Ask This Old House: a wise journeyman is installing CAT5 cables to establish an intercom in an ancient house. (circa 1895)
He gently taps his beater along the line of incission. Then he cleans it up (the plaster skim zone) without vibration. [Off camera -- assume razor knife and beater tapping.]
Next he gets at the support grid. His Kleins (dykes) (2000 series, curved) make short work of that.
More gentle craft work leaves the cavity suitable.
In his case, a LV mount is used to skirt around a totally foreseeable stud -- part way into his work zone.
Routing cable in those old balloon construction homes is a total snap. There is no insulation in the way, nor firestops, nor diagonals.
In sum: they are great tools. They don't cut cheap -- the blades go dull very fast. This is manufactured in so as to keep you spending, spending, spending.
Generally, it's not possible to use generic blades for these power tools. Design patents are still holding off rationalization.
The blades don't lend themselves to field sharpening, either.
Be mindful of tear-out at the tool arbor. This focus of force and vibration is built as a mechanical fuse/ money spinner. Expect your most expensive tool heads to fail at the arbor when you least expect it.
So watch your force level.
Lastly, the el cheapo imports have low power -- and low rates in application. If you have to eat their limitations, then they stop looking so cheap.
The original Fein is still the Duesy. In comes with a very nice (as in expensive) tool case with all of the trimmings. That's not a small factor for a tool that has gazillion working heads. (A multi-master is the exact opposite of a Skilsaw.)
You might want to look into Sortimo, too.http://www.sortimo.com/Cases-and-Boxes.47.0.html
These are very pricy... and have been adopted wholesale by the Hollywood craftsmen. The Fein case is dimensioned close to the Sortimo world, BTW.
In the same vein you might desire FastCap's Kaizen Foam.http://www.fastcap.com/estore/pc/Kaizen-Foam-p13435.htm
It has universal application for anyone who depends upon tools -- and getting to them quickly. With this product you can make your own versions of Sortimo's cases.
You'll want a fast PC to wander around FastCap's website. It's overrun with java scripts -- all of the bells and whistles.
You'll recognize many of FastCap's stuff popping up on Ask This Old House and DIY Network. It's exploding across America.
When you buy your oscillator, think ahead to all of the tool heads that it travels with. Without them, you've cheated yourself.