Personally, I thought GMC was a pile of crashy shite that only existed because they had to have SOMETHING.
In general terms, most people don't want external viewers for silly shit like text and pictures - I just want to look at the damn file. It gets tricky when you try to integrate a web view; my gut reaction is that I like it, but the navigation between web view and filesystem has got to work properly.
Seamless browsing from local to remote filesystem is still arse everywhere. No-one—but NO-ONE—has got this right, and that includes Apple.
I also don't see why I should be starting up a special application just to bung a few files on a CD; after all, we don't have a special gfloppy program to use floppy disks; what's different about a CD?
Also remember that the two main file thingers, Explorer and Finder, are themselves very far from perfect. Explorer has a horrid tendency to lock up when you're doing network things or have a dodgy CD; Finder likes to go BOOM when you're not looking.
Both have huge piles of UI issues, too.