Anne McCaffrey's Dragonriders of Pern series was the first of its kind that I ever read. When I was old enough to explore my parents' bookshelves, I quickly discovered the series, as well as a good number of her books based in other worlds.
When she starting bringing out books co-written with her son, I have to say I wasn't a massive fan (I'm talking about Dragon's Kin, Dragon's Fire, and Dragon Harper). The characters weren't ones I loved, they weren't based around the dragons as much, as I just felt the books weren't as detailed and rich as the earlier ones.
I recently re-read Dragon Harper, then discovered (a) it was better than I remembered, (b) I was a bit behind on the series, (c) my library had the next two books, and (d) I had study to procrastinate! I've just finished Dragonheart and Dragongirl, and apparently just in time, as two new books are scheduled to be released this year.
I did enjoy those three, although that may be the procrastination talking. I won't say I'm happy just to have new books coming out, because I would like an awesome series to stay that way, and we have certain standards to maintain. I don't love them like I love the earlier ones. I think they're doing a good job of exploring different aspects of Pern, while remaining true to most of what's already established. I have to say the theme of Dragongirl was absolutely flogged to death, so I would wish for a bit more subtlety in future books. Overall, however, as most jacket cover comments seem to say, good additions to the series.
(PS: If you have no idea what I'm talking about here, go and find yourself a copy of the first book, Dragonflight.)
(PPS: Exciting news: apparently, Dragonflight is going to be made into a movie!)