Well, I've used Movie Maker and the latest iteration of Roxio's Creator. While Creator 2009 is a lot less buggy than previous versions I've used (specifically, 8), all of the apps that I've ever used for editing haven't been the most stable apps I've ever run. The old Roxio crashed on me all the time, especially when trying to use the MyDVD module to create a DVD with menus, music, et cetera. I've only had one or two annoying and inexplicable crashes with the latest version. I have no experience with any of the Adobe products and because I just tinker with videos shot on a digital camera, buying an Adobe package specifically to edit video isn't in my budget.
I suspect part of this is the proliferation of formats, many of which are proprietary and don't play well with much of the available software. Anecdotally at least it seems as though many people have issues with Apple's QuickTime format, especially on PCs; oddly enough, that's the only format my Kodak camera records video in, so I have to use Roxio to convert any of those videos to something else--there really is no free software that does this, except for some dubious and buggy items you can find on download.com.
I don't think (if I recall correctly) that Vista's built-in apps can even handle .mov and similar formats. So maybe Gnats3 is right--maybe it comes down to too narrow a consumer base for people to produce really good software that works well for a variety of users.
For what it's worth, Roxio's website says that Creator 2009 can burn AVCHD to HD DVD (and I guess Blu-Ray discs too, if you buy the extra plug-in). Honestly, I think it's a bit ridiculous that they charge an extra $30 to burn to the format that "won the war," while you get HD-DVD out of the box!