After four months of ownership, I'm very satisfied with the ML system due to the strong clarity and sound imaging. Although the bass may not be as hard hitting, occasionally giving too much mid-bass - it still packs a decent punch. It all depends on what you are looking for. Good balanced sound overall (even at higher volumes), but not the obnoxiously door rattling aftermarket boom of a true boxed subwoofer. One thing to note, is that the sound source is extremely important as you will detect more imperfections in low quality music (some radio sources, low bitrate sampled MP3s, and especially satellite radio).
The link I provided above answers the size question (you do have to convert from mm to inches if you're using imperial measurements), but there doesn't seem to be any documentation as to how the power is distributed from the amp.
Here is how I think it breaks down:
I would envision that the subwoofer has a dedicated channel at 150 to 200 or possibly more watts (since it is capable of making all the mirrors shake with no issues) and the rest is pretty much split evenly by the other channels.
UPDATE (probably only valuable to people still considering buying).
We picked up the IS yesterday and went for a 200 km drive. We got the 350 F-Sport Executive package (we're in Canada, not sure if options are different in USA/globally) that includes the Mark Levinson stereo.
The sound quality at high volumes is excellent. You really can crank the stereo to near max volume without distortion. Very clear highs and mids. However, the low-end is seriously lacking and if you listen to Rock, Country, Hip Hop, or Electronica... you'll want an aftermarket sub.
We're a little disappointed in the stereo since we upgraded to the Executive package for the ML stereo, and for the extra cost it simply wasn't worth it. It's worth mentioning that the Executive package came with radar cruise control, blind spot monitor, heated steering wheel, etc. If you find value in those then you may not mind the stereo so much.
This isn't to say that the ML stereo is bad. For us, and the kind of music we listen to, it lacks in the low-end somewhat. However, compared to other vehicles stock stereos it sounds great. If you don't want a lot of low-end or deep notes, you'll be just fine with the ML package.