This is a blast from the past - I haven't read any Pratchett for years. We went to Dursley over the bank holiday, and I came back with a number of books including half of my Pratchett collection (which was rather larger than I remembered), and a good deal of nostalgia. So, I thought I'd work through them in order, eventually.
You can tell it's his first go at the Discworld. If you're writing a new book about a magical disc-shaped world, you're probably going to make one of the main characters a magician, and make the action culminate on the Edge of the World. So, this is what he does before settling down into the rest of the series of books. There's a lot of mickey-taking of the fantasy genre in general, which is replaced in later books by just concentrating on constructing his own world, and if anything, taking the Michael from the world in general. It's certainly difficult to see how the series will eventually pan out, since while there are many elements which are repeated in later books, there's a fair amount which gets dropped, too.
In general, it seems a harsher world than the later books. Perhaps it's him settling into his style, perhaps Pratchett is just increasingly unwilling to kill protagoists if they may usfully appear in sequels. The overall book structure also has that 'just starting' feeling. It's more a collection of interlocking short stories than a conventional novel. This is no bad thing, giving a little bit of variety, and stopping things getting bogged down, but still feels a little odd.
Fundamentally, it's enjoyable enough, but I wouldn't recommend it as something for those new to the Discworld. Much better read after a few of the others, once you have a background, and an interest in how the series started.