I've been using one for a few months and found it to be a very good club. I agree with DCB in that I found it not as long as my previous driver (I've probably lost about 10yds), however, my accuracy with it has improved, I'm hitting more fairways than I used to so it has improved my game.
The weight system does take a bit of getting used to, but well worth experimenting with. I got a friend to change the settings from draw to fade without me knowing which was set. It does work, it won't cure a raging slice or hook but it will allow you to set up for a particular course.
Most importantly, don't just buy one off the shelf, go and get fitted properly, preferably at one of the Mizuno National Fitting Centres. I had a good idea what shaft I needed before I was fitted, what I ended up with was totally different and much better.
I've since compared my idea of the ideal fitting to what Mizuno recommended and there is quite a difference. I'm a big believer in having clubs properly fitted, even down to my putter. It really does (IMHO) make a difference.
I chose the MP 600 because I have a personal preference for Mizuno clubs, no doubt if I had been fitted properly for a Calaway, Taylor Made or whatever make I would have seen an improvement, in the end it's down to personal choice.
Whatever your choice, best of luck with it.
Brom