Firstly, I think that this is a great product! The whole book is a joy to read. Weighing in at 240 pages, in large format, and hard backed - it's a lovely item to own and look through, even if you never play the game. It is richly illustrated with art from the comic books, as well as some lovely painted miniatures throughout. Mongoose definitely has the right idea about presenting nice examples of their figures, and their studio painter is excellent.
The book covers simple and advanced rules, then campaign play, and finally some fluff/background. The basic premise is that you pick a gang type (from Judges, to Demonic Cabal, to Sky Surfers), and engage in a small scale skirmish. There are various missions and objectives detailed in the book.
The game mechanics are simple and easy to master. It's a basic gang war, I-go-you-go affair - nothing groundbreaking in terms of dynamics. But the rules do feel very Judge Dredd: with special actions allowing arrests, hiding, etc. Overall, very easy to learn but definitely geared towards "beer and pretzels" type play. The whole thing feels very early Rogue Trader, but with much better presentation. It's a nice break from 6th edition 40k, where the rules seem to have become so bloated that it takes forever to get through a game....
Heroes deserve a separate mention. They have "talents" - special skills which separate them from minions. The talents add an interesting extra dimension to the game, although I found finding them in the rules cumbersome at times (especially for experienced heroes like senior judges: they quickly acquire a huge range of skills).
A point worth making about Judge Dredd is that it is very much a skirmish level game. A starting Judge gang might have 3 figures, whereas a gang of Cursed Earth Desperadoes may have half a dozen to ten. Every figure really counts!
A mixed observation is the introduction of mercenaries. I found this mechanic unusual, and it might well reflect the manner in which Mongoose intends to expand the game. As noted, you start by picking a gang. This gang then has access to "reinforcements" - usually a long list of extras figures, often with more exotic weaponry and equipment. The idea is that when your gang faces another, bigger gang, you can induct the difference between credit values for the gangs but in mercenaries. These mercenaries can't gain experience, but otherwise act as part of your gang. The difficulty with this approach is that certain miniatures seem to be core gang members for some gangs, then mercenaries for others, and the decision as to who is a gang member and a merc is pretty arbitrary. Some mercs are really just typical gang members but with particular talents or weapons. However, the benefit of doing it this way, I suppose, is that Mongoose can release figures on an ad hoc basis: there are already lots of PDF files on their website, and I could easily see new characters being made available in this way.
Finally, we come to the price. This can be a deal breaker for may war gamers, especially with companies really upping their prices recently. I certainly found the price of £30 for this product very reasonable - it's such a nice book, that I could justify the price. Even better, many websites have it on a discount - Amazon lists it for £22, for instance.
Overall, Judge Dredd: The Miniatures Game is highly recommended!