*cough* Anyway, moving on....
The subject material is, especially for me, fun. We get to see Dick Simnel become the Disc's first engineer, relying upon his trusty slide rule and his precise measurements, so that he can develop the first steam locomotive. He then comes to Ankh-Morpork and Harry King, one of the most wealthy men in the city, and with the cunning help of Moist they begin construction of probably the greatest project the Disc has ever seen. Of course, I wouldn't be doing my job as a reviewer if I didn't find something to constructively criticize about this novel, although personally I do worry that I'm turning into one of those fans that isn't entirely happy with new works because they're different.
Aside from the railway story, Raising Steam also deals with an internal religious schism among the dwarfs. The grags and the other deep-downers, which we had seen in Thud!, are causing trouble once again and have begun attacking people they consider to be not "truly dwarfish", as well as burning down the clacks system that has become so important for communication across the Disc. The Low King of the Dwarfs is particularly frustrated because it threatens to undo all the good work he has done with Lord Vetinari and Diamond King of Trolls. The plot is....interesting, but the jumping between plots is rather confusing in the very early parts of the books. Pratchett has written books, Hogfather being probably the best example, where multiple narratives occur throughout the book and eventually become sewn together in the novel's finale. It probably makes more sense on re-reading, but it makes the book feel extremely disjointed in the first hundred or so pages of the novel.
Personally my other big criticism of the novel is that things involving the railway frequently happen off-camera so to speak. We get to see Dick Simnel bring his idea to Ankh-Morpork, and working out the initial details with Harry King. Eventually they build a line from Ankh-Morpork to Sto Lat and it's opened with great fanfare, and then daily traffic between the two cities becomes common place. Eventually the railway's lines expand across the continent, heading towards distant Uberwald, and we see only slight fragments of this. Dick Simnel develops new technologies, builds new engines, and an entire railway culture is developed. Yet we see much of this in passing, if we see it at all. It's probably because I'm a Rail Enthusiast myself, but I really wanted to see how the railway was going to develop on the Disc in much more detail. Perhaps it wouldn't have been as interesting to outsiders, but I think Pratchett made even the post office seem interesting in Going Postal, so I think he could have risen to the challenge.
Aside from my complaints, is the novel good? Oh definitely. If you're already a frequent traveler to the Disc then I'd highly recommend you hop on the Ankh-Morpork and Sto Plains Hygenic Railway. If you're new, I'd suggest starting earlier in the series so you can build up some steam before tackling this novel. (And no, I refuse to apologize for that pun.)
....I like trains.