Of all Garth Ennis' extended runs on characters (Dredd, Hellblazer, Hitman, Preacher, Punisher), Judge Dredd is for me his weakest by far. The stories come from early in his career and in a lot of ways Ennis is still learning how to tell a good story. However, unlike Hellblazer, another of his early works, where the occasional clumsiness did not obscure the brilliant wit and invention that made so many of those stories so much fun, most of his Judge Dredd stories revolve around Dredd being incredibly tough and killing lots of people. This is occasionally taken to ridiculous levels - Dredd's killing of two BILLION people in the previous story is mentioned in this one, and in this story Dredd survives being shot several times in the side and crucified without food and water for seven days after which he tears himself free and continues to wander around killing bad guys with no apparent loss of strength or stamina. Things like this take away from the believability of the character and make it difficult to feel any tension.
Compared to John Wagner, who created the Dredd character and wrote Dredd before and after Ennis, Dredd's character under Ennis loses the subtle black humour and obsession with the law to the point of monomania, what humour there is over-the-top and Dredd's character is interchangeable with many other comic book tough guys who kill at the drop of a hat.
Despite all the negativity above, there are good points about Ennis' work on Dredd, and Goodnight Kiss is far from his worst. It tells a mildly entertaining story with lots of over-the-top Ennis violence.
In summary, if you've read a lot of Dredd and want more, you should pick this up - mediocre Dredd is still better than 90% of comics out there. If you're new to Dredd then I would recommend buying one of the Judge Dredd collections by John Wagner instead.