Caution -- I used one of these and I think it led to bad habits which eventually caused a bit of RSI.
Before the model M I typed very lightly on the keyboard, but once I started using the M I began to press a bit harder in response to the positive clicking sound. Neither my speed or accuracy changed as a result of using the M.
Next thing I knew, I'd pound the keys on any keyboard, regardless of whether it had positive clicking.
This led to increased strain on my fingers and wrists and eventually I had to re-teach myself how to type lightly.
Now I type just as I used to before the M. I caution anyone considering getting one to be aware of the overall amount of physical pounding his/her fingers and hands are engaged in, and to realize that it can create additional tendon fatigue and bad ergonomic habits.
"I'd pound the keys on any keyboard, regardless of whether it had positive clicking."
My experience is the reverse, I have a couple cheap mushboards at work and the variability of force means I have to hit each key exactly the same each time (bad) and have to pound the heck out of it because the A key takes about twice the force of the F key (however small) and tiny variations in striking angle results in dramatic change in force required. So my hands have to be in exactly the same position every time and I have to pound the heck out of it. Luckily this is a secondary machine I don't have to type very much on!
On the other hand my model M at home is smooth as silk so I can microscopically vary my wrist positions with no change in force required, and decades later the force required is still more or less constant across all keys and lower than my mushboard at work.
Twenty-years of typing on model Ms here - no RSI. Then again, I've been playing the piano for 35 years, so maybe a well-ingrained slightly-rounded wrist position and controlled striking of the keys has benefited me with both the piano and typing. :)
But I agree - one doesn't actually need to type any harder on a model M, in fact - like the piano - speed and stamina increases if you use less force.