I was riding the new M2 toy 400PRO in this mornings class at M2 and I thought I share my thoughts, well knowing that this was the last time I rode the thing (because you will all jump on it now).
While it appears to be similar to the 300 in physical appearance, the resistance dial is a mere dummy and you control the wattage using the little joystick on the right of the Joule head unit that you never used before. Pressing it down cycles between 'gear', 'target grade' and 'target power', while nudging it up or down moves the set value up or down (you guessed that). M2's recommended warm-up is on 'gear', where a higher number indicates an easier gear. I would find a sweet spot and then go up 2 (easier) for the cadence and down 2 (harder) for the stand. Moving into the main set of intervals it's pretty simple. Put it to 'target watts' and dial in the watts you estimate you can hold. Note that the unit will make you overachieve this target by about 10-15 watts on your average (ie you set it to X watts and you end up averaging X+15watts). A clever trick is to keep the 'gear' in a high gear so that when you come off a hard effort you can push down on the toggle switch and it becomes easy to pedal/rest. This works great, but there is a slight delay in ramping up and down and it's a bit tricky for very short intervals (ie 20s). Also the watts you are actually pushing fluctuate quite a bit (set point X, will make you push X +/- 20 watts), you can either like this as a rode-like feeling, or hate it. Your choice. I like it.
Overall fun experience, give it a try