I'd give it a 7 out of 10. It was fun, but not too exciting. I found my attention drifting about 75% through; I was watching it but thinking about other stuff.
What I really liked was the imagining of the discworld - the set designs, characters, and general appearance of everything was fantastic (well, some of the CGI looked a bit cheap but nothing too bad). I really liked how the characters were portrayed in the most part as well.
What fell flat was the overall storyline. Partially this is the fault of the source material. The Color of Magic is episodic and was written before Pratchett really figured out how to pace his material. But the tv adapatation, in my opinion, highlighted the pacing problems rather than smoothed them down. Partially it was the decision to fit two books into a 2-part, 4-hour miniseries; so while yesterday was "part 1", it was actually the entire novel. I don't mind so much the large chunks of plot being cut off, but the story ended up so truncated that the narrator had to come in often and tell us what was going on. And - though maybe this is because I know the book - I occasionally felt there was way too much exposition, and some of it repeated needlessly. Often the narrator would say something and then a character would repeat it later. This was partially borrowed from the book which does the same thing, but in the book it is often a way for Pratchett to stick in more jokes, and partially a sign of his inexperience at the time that he later shed.
Also, the humor didn't work quite as well as it could have. It was often funny, but Pratchett's humor tends to be composed of occasional very obvious, signposted jokes with more subtle jokes in between them, balancing things out. Again, he didn't quite get the balance right in his early books, but the adaptation went the way of just ignoring more subtle stuff in favor of very obvious humor.
I think partially the issue was that the show was aimed more at children than at adults; I have a feeling that if I was 12 years old, I would have really loved it. But as a 32-year old Pratchett fan, I found it enjoyable but not something I would buy a DVD of to ever watch again.