Sequels are a touchy thing. I love sequels if they're done right. But they can make or break the franchise.
Look at Resident Evil, for example. 1, 2 and 3 were good, but then the sequels started going down a slippery slope. The storyline, as of now, is completely butchered, and RE went from my absolute favorite series, to RE5, which I'm not gonna rent, and am isolating myself from as much as possible, because I don't want to be exposed to its storyline.
Bottom line is, I like sequels, but they need to know when to end. Like, look at Onimusha, although Onimusha 3 wasn't the best in the series, it was still good, and it was said to be the last. And then they release Dawn of Dreams, and it sucked. So O3 really did need to be the last.
Many Metal Gear fans gave up on it after Metal Gear Solid. Personally, I loved the series right down til the end. But as far as I'm concerned, Metal Gear Solid 4 IS the end. And whatever comes afterwards.... If it sucks, I don't care, I just won't play it. Because the storyline has concluded nicely with MGS4.
But in general, I'm in favor of sequels. If they never stop making Ratchet and Clank games, I'll be a happy man.