I take care of a university jewelry/art metals studio with about 15 flex shafts (and spent several semesters in the same studio as a student a few years before). I also have a jewelry supplier branded clone in my home shop. This is what Ive learned from my experience and talking to the professor and several visiting artists.
The flex shafts are used hard by 30 to 50 people each semester, half to 2/3 of which are beginning students. About 5 years ago we added 10 Foredom TXHs. The other 5 are a mix of Otto Frei and Rio Grande branded Foredom clones and 1 or 2 of the genuine Foredoms. The oldest of them probably go back to near the start of the metals program in the mid-1970s.
Ive had very few problems, one of the old ones Ive retired for parts or as an emergency replacement, but it is still functional. Since a lot of the use in the studio is drilling with small number size drill bits they all have drill chuck handpieces, but we have a couple hammer handpieces also. The handpiece is responsible for most problems I deal with, they get dropped and banged around, people over tighten the chuck and the gear teeth that close the chuck wear eventually, especially if you are sloppy with the chuck key. The good thing is that replacement handpieces are fairly cheap. The inner shaft can break, especially if the shafts are bent too tightly, its fairly rare, but I keep a spare on hand. Now and then Ill have an issue with the foot switch, so far nothing a bit of cleaning and adjustment hasnt taken care of.
There are now a couple of different shaft/handpiece configurations. The Foredom TXH uses a larger diameter (and longer) shaft that has a square section that fits the socket on the handpiece. At least so far there isnt the selection of handpieces and I dont think any aftermarket handpieces for the TXH. The older style seem to be universal, no matter the maker, a keyed metal piece that fits a keyed socket in the handpiece. There is a lot of choice of style, quality (and price) in handpieces, thinner ones, ones with a spring where the shaft joins the handpiece for more flexibility, drill chuck, lever quick release collets and hammer handpieces. There are also aftermarket foot controls, most people like the heavier cast iron cased versions, they stay where you put them. Handpieces and foot controls run from inexpensive clone replacements to, especially for handpieces, high quality imported (Badeco or Technique) that cost more than $200. It takes only a few seconds to change handpieces.
Foredom is Foredom, the gold standard, but from my experience the clones are nearly as bulletproof, if not quite the feel and quality in the handpiece and foot control. Buy what fits your budget, the most powerful motor you can get and you can upgrade the handpiece and foot controller later should you decide to.
Todd