I carry a Jumbo as EDC. I found the Fatboy was a tad small for me and the Jumbo is just about right. I think a lot of it depends on your use and what you are looking for.
I've added the following to mine:
1) Jumbo Rollypolly on rear upper strap mount point.
2) M2 pouch on front strap mount point.
3) Standard Rollypolly (sewed) above forward pocket
4) Modified gearkeeper* on strap above M2 pouch
5) I hooked a couple of
stainless "Twisties" into the top webbing so I can clip my keys on.
* Replaced cheezy gearkeeper clip with
this sturdy stainless clip from Berkeleypoint.com.
I carry mine almost always and have rigged it up so it works for about everything. For example, I have the clip on there so that I can clip my dog's lead (
Ray Allen Flat Braid Latigo Waist Lead) to the pack if I need my hands free. The jumbo rollypolly can hold a bunch of balls and frisby at the dog park. I keep plastic storage bags in the standard rollypolly (you can roll up the rollypolly with them in there) and then open the rollypolly for transporting dog waste until we get to the next trash can.
So for me, it really works out for almost every occation. I can EDC it, and it can accomidate outings like the dog park.
The downsides?
1) If you carry it around in an office setting you will get some comments.
2) It's big enough that you might find it getting a bit heavy.
3) I had to sew on the second rollypolly. No attachment point where I wanted it.
4) No place for 8.5x11 papers.
5) Doesn't fit well into a backpack.
6) The flap doesn't cover well enough for rain protection.
7) I have some minor nits about the drawstrings in it.
Overall, I love it. If only it could be part of a modular backpack system...
-john