I've replayed FFXIII's main storyline several times, and I've replayed chapters 10-13 even more. I've also done a decent amount of challenge play (currently working through all the missions at stage 8 of the crystarium), and I intend to do more challenges at some point.
Yes, it's fun. ^_^
I replayed FFXIII-2's main story once, and I have more to do in the game since I've yet to get more than 115 fragments. I also intend to check out the various DLC fights and scenarios.
Got all fragments twice in XIII-2, and am currently considering quitting my XIII playthrough due to having gotten to a place where I need to farm to keep up.
Unless you're doing a challenge run or working on Treasure Hunter, I guarantee that farming is not necessary. Improving your strategy will suffice. Drop on by the FFXIII board if you want tips or advice.