Documents to Go is a nice enough application, and if I really needed the few other features, and better DropBox integration, I'd probably get it; but the iWorks applications do what I want, and do it very well (for mobile apps). Being an iMac owner with the OSX versions of iWorks also makes it worth sticking with what I've got.
BTW, Box.net is looking like the better cloud storage/sharing solution for iWorks. It has WebDav service, but loses the local file syncing on the computer (for the free account); so not perfect, but better (imo) than Dropbox for these apps.
And it's not like it's an either or choice. You can have both.