Well, I start to understand better how the thing works, especially in respect with its little brother, the Imagic.
One thing is sure, the choppiness problem occuring in slopes steeper than 10% is recurrent...
I'm considering buying (an used) Imagic...

What will I loose ?

-The inertial effect of the fortius electronic flywheel
-The ability to produce high wattage at low rpm, although nothing about the wattage capabilities of each model is described on the Tacx website. I've post a topic about this on the Tacx forum, we'll see...

As far as slopes and power are concerned, the difference between Imagic and Fortius are as follows :
When you get on a steeper slope with the Fortius, the motorbrake just brakes harder and harder till you get to 10% slopes, then you've got that choppiness feeling. Till 10% Speed, slope and Power on the screeen are the same as in "real life".
To solve the Fortius choppiness problem, Tacx introduced "virtual speed" in late 2007. Which means that to a given power your speed will be virtually reduced. You're going harder to overcome the steeper slope, but your speed will decline... It's like if the software decided that a strong wind would blow in your face as soon as you would get on a slope steeper than 10%.

When you get on a steeper slope on an Imagic the virtual speed kicks in as soon as 5%.

Nonetheless, people who have tried both trainers said the Imagic felt "smoother", less choppy...
As (I repeat it) nothing about the wattage capabilities of each model is described on the Tacx website, I wonder shall I buy an used imagic or send the fortius back to my LBS hoping they'll accept a change for an imagic.
I bought the Fortius, cause I thought it could deliver more power at lower RPM, but maybe I'm not able to sustain that kind of power and an Imagic would be enough for me. (I've got a 400w MAP)