okay - so it looks like I get the short straw as the first actual Fortius owner to answer the question...
So...
There are 3 main ways that you can use the Fortius: Virtual Environment, Real Video, and Catalyst.
Virtual Environments:
Think of this as a giant map with roads all over the place. (May be easiest to think of it as riding on an island) There are flat sections, hills, and intersections etc. You can use the software to design any combination of roads that you want and then ride it - or you can use the steerer to allow you to ride 'free-roaming' and just turn up and down whatever roads you like. You can programme it to include robot riders which may or may not be past versions of yourself on the same circuit. You can also use this to copy in other people's rides and race against them and more recently, you can also go full multiplayer and race anyone in the world at any time - live. There is also a virtual environment that is set on a velodrome.
Real Video:
This is the cool feature as far as I am concerned. They have produced a large number (and climbing) of videos of major races and climbs etc. You can elect to do the entire video start to finish, run one of their pre-set segments, or even cooler, build your own video from the main video by splicing together segments. (Just think, you could do Alpe d'huez and then when you get to the top, just go straight into another one and another one....). If you don't want to sit on the bike rolling down into a valley you can just chop the valley out of the video and go from climb to flat to climb, for instance.
I have in my collection: Milan San Remo, Majorca, Alpe d'huez, Galibier, Ventoux, Flanders, and several others that I cant think of right now.
Catalyst:
Think of catalyst as being like a traditional ergo training environment. You can set up a view with live data on screen showing all parameters such as HR, power, Cadence, Gradient, etc. You can use this for interval training, etc in pretty much any way that you want to.
How it works:
For Virtual riding and Real videos, the motor brake works based upon the course you are on. The device knows your weight inc bike (because you have told it) and the brake adjusts resistance based upon the gradient you are on. This works very well up to gradients of 15% plus. When the gradient is negative, the brake back right off and when you are really flat out descending, the brake actually pushes the wheel. I find the resistance from the unit very smoothe and progressive. The unit is quite quiet for what it does, too.
To answer some of the questions directly:
The device is very stable and you can sprint on it.
You can make your real life video based sessions run for several hours if you want (or 15mins)
It can do any interval workout you want (you dont programme it for that, it just reports what you are doing)
Standalone mode is a bit sucky. It can work but I wouldnt really recommend it unless you are truly desperate.
The negatives: Its a complicated device with a number of cables running between the bike, it and the PC. This means it is best if it is permanently set up somewhere. For me the biggest negative though is that in an attempt to create a safety device, Tacx have set it so that cadence must be above zero in any 5 second period. This means you cannot do an effort and pause pedalling to towel off etc. If you do that, the brake suddenly fully engages to stop the wheel. The idea was an okay one, they were thinking you may have passed out or left it unattended (which would be very dangerous with kids in the house for instance) but the implementation sucks. For starters, they use a WIRED candence sensor. So even if you have the best and quickest set up method, you still need to faff around setting up the cadence monitor every time you put your bike on the unit.
However: I have created a workaround that fixes this. I will not publish it online though so if you really want to know what it is, PM me.
Overall, I would happily recommend the device. When I lived in London, this thing was the only riding I did for a full year. Even after that long on it, I still enjoy doing sessions on it sometimes when the weather sucks.