Eric by Terry Pratchett
Highly amusing, and the first book where I've really enjoyed Rincewind. Also it's fun to branch out more than is normal for the Discworld, lots of different places including the afterlife. Getting away from Ankh-Morpork is a worthwhile change in pace. The take on hell and demons works particularly well--bringing up a number of implications and afterthought to the traditional monotheist cosmology, while at the same time dealing at it without the overly bitter edge common to a lot of religious deconstructions have. It never achieves the level of quality or sheer enjoyability of Good Omens, but in its somewhat meandering comedic way this book works effectively.
Unfortunately, by the same token, the plot of 'wander around to different places while satirizing Faust' wears a bit thin, and none of the characters beyond Rincewind feel truly real. Ultimately the piece feels lacking in internal relevance--the characters and the universe aren't substantially altered from where they started the book, and the main focal points of the story feel rather disconnected from the main Discworld. That's not a fatal problem in itself, but looking back it does serve to weaken the force of the story a bit. It could be a case of expecting more from the story than the whole setup aimed at, but for all the amusement and sense of energy there also look like a number of missed opportunities.
Worse than: Going Postal by Terry Pratchett
Better than: Wyrd Sisters by Terry Pratchett