1. Does it still seem to come on too early?
Not that I have experienced. It still let's you step the back end out enough to smile, but not enough to make me think about it snapping around.
2. Does it only help in limited situations? If so when?
MDM is useful when you only want/anticipate a little slip, and you do not want the computer cutting power to the rear wheels, possibly causing u you unload your suspension
3. Does it come on too late?
I suppose this could be subjective. I have only had a small hand-full of times when it seemed to kick in when I didn't think I was anywhere near losing it. This was back over the summer, but since then I have been pleased w/ it not being intrusive. I was never able to diagnose why it came on then. A fellow E92 M3 driver had the same thing happen to him on the same stretch of road, so we chalked it up to gravel that we didn't see.
4. Is it more abrupt than the normal DSC when it does come on? (Or is it smooth and seamless sending you where you pointed?)
It's abrupt in the sense that it probably saved your ass, and you know it. Generally you will realize that you over-drove your skills before it kicks in anyway. You definitely will know when MDM kicks in.
5. Will it ignore wheel slip in a straight line and not shut down the throttle?
It seems to ignore wheel spin while accelerating in a straight line in my experience. This is occurring more often for me since I have been lazy in replacing my rear tires, but if I put MDM on and accelerate hard onto a highway it lets the wheels spin until they hook up without the power loss that a fully on DSC initiates.
Bottom line: is it of marginal utility and you'll turn it off anyway, or, is it a very worthwhile feature?
IMO I find it very worthwhile, and I spend most of my driving w/ in MDM mode. I'm a decent enough driver to know that I am not decent enough to leave it off all the time, and I use it somewhat as an instructional tool to learn my limits, how I got there, and what I can do in the future to improve. Also, it allows you to change the quickness of the wheel inputs (I am pretty sure this is only available w/ M-mode, but I may be incorrect), and you definitely notice this when driving the vehicle aggressively. My E46 M3 did not have a MDM mode (didn't know you could get it on an E46), I just had a sport button, and DSC off, so I can't speak to improvements over an older system. IMO it is a worthwhile option for most people, especially those that use it for both a DD and track car. That being said, I would prefer that it just came set-up in the more aggressive MDM mode full time, and the "softer" normal setting was eliminated. After all, who buys a M car for it's supple ride?