The only really useful apps are MS's (like OneNote MX) and Kindle.
I have to say I'm really impressed with OneNote MX, despite not being sure what MX means. I find it extremely easy to use, and while the radial menu is something to get used to on a mouse and keyboard (insert "no comprimise!"-DeathByVisualStudio-quip) I can imagine it being great when using touch.
If there's one example of productivity tools being possible in the Metro/Modern/Whatever design, this is it.