This book is one of the best crossovers I've seen. Judge Dredd is my favourite comic character, and everyone likes Batman.
The book is divided into three parts, Judgement on Gotham, The Ultimate Riddle and Die Laughing.
The first part sees Dredd's evil nemesis Judge Death use a dimension belt to warp into Gotham and basically kill everyone (long story, Batfans, read Necropolis) and Dredd has to sort it all out.
The second part shows Dredd and Batman warped into a parallel dimension to do battle with mean badasses from across the universe.
Die Laughing, sees the Joker join forces with the Dark Judges... Judge Joker. 'Nuff said! In my opinion the best of the three stories.
The artwork is second to none in all the stories, and across all of the stories the tension between old Stoneyface and Batman is portrayed perfectly. Both stand for justice but have very different opinions on how to achieve that.
As I've said, I bought this as a Dredd fan, but Batman fans like it, and if you're new to Dredd then this is as good an introduction as I can think of.