I've read through most of the meat of the book now and I think the rules are very well done. I wouldn't mind seeing an alternate PHB with these rules incorporated so that you could just bring copies of this book rather than a whole book of what would function essentially like errata. It really is the "3.75" that would make the game more enjoyable in a lot of ways. In many ways, it mirrors a lot of what WotC appears to be doing to make 4E more playable.
Specifically, I like the 20 levels of spells, which is really a core concept to the book. It really simplifies the magic system. I'd imagine that this is similar to how 4E is going to work the spell slot system, and seeing how Monte implemented it, I can only imagine that the 4E version of the same will be very player friendly. I also like how healing, death and dying, and the magic using classes were reworked.
The only things that I feel should be addressed that isn't is attacks of opportunity balancing every special combat move, and simplifying grappling.
I also feel that Monte could have really competed with 4E with this product if he would have toyed with class progressions in a way that would extend the sweet spot out further. Of course extending out the sweet spot is already written into the core rules in the optional rule that would have you to double the XP threshold to gain each level.
Finally, I also like the conversational tone throughout this book. Not only does he present the new rules, but he does so in a manner that's just one gamer to another, and then he often explains his thoughts behind the design decisions.
All in all, I give this a 4.5 out 5 stars.
As an aside, I found it ironic how Monte alluded to the notion that he had planned on stepping out of RPGs altogether, but couldn't resist coming back in to keep tinkering with it. I can completely identify with this as I considered myself out of the design game just a couple months ago, but after about three or four months of having nothing on my plate, I've gotten back into it myself. I've taken on a project of great personal interest for a small publisher that has done some kickass stuff in the past, and I'm having a great time doing it again. No further comment than that, other than that I think once the game design bug has bit, the infection never really goes away. It will go into remission from time to time, but when it comes back, it hits with an all consuming fever, driving you to obsess over it until the project sees completion.... or something like that.