Yep. For road use, they'll be adequate. For off-road use, they'll just plain suck. Let's not forget, windchaser, what the Felt Nine-Flow is designed and made for. Sure you can replace the fork, replace the tires, replace all the parts and you'll make it into a commuter bike but why? There are lots and lots of bikes out there that have the right fork, the right tires, the right shifters, rack mounts, fender mounts, etc. at a pretty good price without a whole lot of parts swapping. You can keep the Nine Flow as a mountain bike that is great fun to ride off-road...you can even look for trails to ride it off-road on the way to work...while using the other bike for commuting and/or riding fast.

Upgrading can get costly. Upgrade to make the bike do what it was built to do better. Don't 'upgrade' to make it into something it isn't.