I have an uneasy relationship with sequels. I have written them. I have read them. Yet I'm not sure that the sequel is generally such a wonderful thing, artistically speaking.
I've read plenty of sequels. As a reader of fantasy fiction, I am, almost by definition, a reader of triolgies and sequences, cycles and series. From the three parts of the Lord of the Rings, to the extended runs of most urban fantasy authors, sequels are unavoidable. They have their moments, too. Over time, authors grow into characters, so that they get more from them and start to understand the detail of them better. I would argue here that Jim Butcher's current Harry Dresden is a very different character to the one at the start of the series.
I've written them, too. Well, that's obvious. I'm a ghostwriter. I write a lot of YA and romance, and sequels are commercial. The reader knows what they're getting, and if they liked the first one, then there's a much better chance that they'll buy the second one. There's even a trend towards giving away the first book in a long series free, to get people hooked on it. I've even written sequels for my own work, in the form of Witch Hunt, and in a never quite right sequel to court of dreams that sort of makes fun of vampires, and which probably won't see the light of day.
My current thinking is that sequels have inherent problems to go with their benefits. Ones that can be quite hard to overcome. You see, if you've written a complete story in the first one, then you've already had the main character undergo a meaningful character change. So what is there to do in the second one? And the world... well, if you're doing it right, the world is a reflection of the themes and needs of the first novel.
It is, I suppose, a little like cooking. You create this amazing dish out of the best ingredients you can, blending them together perfectly... and then you want to try to make something else out of the leftovers. It isn't impossible to do it as well, or even better, but it isn't easy.