Kingmaker is a good module, but Witch's Wake and Shadowguard are unfinished fragments that aren't worth buying yet.
Kingmaker is a fairly short, somewhat open-ended adventure that you can get up to 15-20 hours out of if you complete all of the quests. The game offers four highly entertaining and well fleshed out NPC's to join you, of which you can pick two. There is also an additional character that takes the form of a powerful magic weapon that you can wield throughout the module. The module has a nice short story to it and it wraps up nicely at the end while leaving room for a sequel. I found that this module exceeded my expectations, as it contains some of the best single player content of NWN yet.
Witch's Wake is just chapter 1 of what looks like it could have been the coolest story ever made. This premium version has some extra goodies like voice acting, while the free version on the NWN Hall of Fame is a more stripped down version. It has a very Planescape-ish vibe to it and a lot of mystery and role playing. Unfortunately, it ends after about one or two hours abruptly, and no additional chapters have been announced since the original was released four years ago. I would advise you to just get the free version unless you see an announcement that this story will be finished some day.
Shadowguard, like Witch's Wake, has enough material to entice you, but since 2004, there has still been no announcement that it will be finished. Since playing one chapter of a story is a lot like reading just the first third of a Harry Potter book, I recommend passing on this one unless Bioware announces that they will finish it.
Gameplay and graphics-wise, you'll be in very familiar territory here. Perhaps too familiar. If you're tired of seeing the same blocky and repetitive tilesets after playing other NWN modules, then you're really going to be sick of seeing them by now. This engine hasn't aged gracefully, and the fact that one cave or forest looks like every other one doesn't help. The gameplay offers the same gimped party system that is functional but not exceptional. For $20, this isn't a great deal, but Kingmaker is worth buying. If you can find this collection in a bargain bin and you really need more NWN single player gaming, then it might be worth it. But first, consider buying Kingmaker off of Bioware's store.