I use Quickoffice only because I can import email attachments via 3G, I've heard that Docs To Go is better for word docs though, and I'll probably switch to Docs To Go if they add importing email attachments via 3G.
I've used both, and ended up deleting QuickOffice and sticking with D2G. It has a much better interface, and more features for editing docs & spreadsheets. It lacks the "cloud" functionality of QO - which allows access to a MobileMe iDisk. But I didn't renew my MobileMe subscription, so this no longer affects me. Dataviz are working on a cloud solution for D2G, but I don't know how it will work. I'm hoping it will be more "open" that QO and not be restricted to just iDisk. Personally, I hope that either Dropbox or JungleDisk will work with it.
The D2G desktop app is actually pretty good. It dead easy, and quick to sync your files. Once you've done an initial sync, it just sync the files that have changed on either the desktop or the iPhone.
The next update will apparently bring some big improvements to the PDF reader in the app, allowing for bookmarks, password protected files support, tap to zoom and full screen. This will be a big help if you read large PDF ebooks/documents.
Anywhoo, I'd definitely recommend D2G and I think it's still on sale until midnight tonight.