Possibly one of Pratchett's best books; possibly an unmitigated disaster. Personally, my opinion leans toward the former, although it all hangs on to what extent a) you're willing to accept multiple subplots with radically different atmospheres, and b) you're willing to accept secondary plotlines as supporting material for the main content, rather than demanding that every plotline reach the same high level of quality.
Personally, I found it deeply moving and beautiful, and also funny and fun. This is probably Pratchett at his most serious and philosophical (it's primarily a meditation on death, and secondarily a consideration of the dehumanising power of modernity), but it's also Pratchett at his most bizarrely, comedically, strange.
The result is, in my opinion, a wonderful book, and probably the best Discworld novel he had written up to this point. Others, however, will find it rubbish.
Fortunately, it's under 300 pages, so if you like Discworld and have read the previous books you may as well give it a go.My fuller review, as usual, is over here.