I am wondering how many guys have fuel paks and do you really use them? If so, how often? Thanks.

Only time I thought about having one was on long rides where someone may run out of fuel. My cousin ran out once, but he didn't have his tank toped all the way off when we left.

We almost always use some of ours. I just feel more comfortable having it and then we can also plan for more remote trips. We each have 2 - 1.5gallon packs.

Depends on what you ride and where you ride. My machine is pretty thirsty so yes, I do use it. Be aware the new style flat packs have a terrible seal around the cap and will probably leak if you lay them flat. It took several calls to the manufacture to get a different style cap that seals better, but it's still not perfect. If I had to do it all over I would carry a couple 2 gal cans on the front rack and be done with it. Cheaper too.

I have the large (3 Gal) fuel pack. I really like it! Its mounted under my rear rack-pack. I use it fairly often since when I ride its usually long days. I have it setup so I can get it off in a couple minutes. I haven't had any trouble with the cap leaking (once I figured out how the seal was supposed to be installed)
We just got back from our annual "spring ride" where we rode 8 hrs the first day, 13 hours, 12 hrs, 10 hrs and 7 the last day. We put on about 370 miles total. The first couple days we used quite a bit more gas than usual because in many places in the woods there was 8,10 maybe 12 inches of sugar snow. I normally never get less than 20 or 22 mpg, but at one point we were on a trail, going down hill and all 5 of us were stuck. Not one of the machines was stock either. All of them have either oversize 589s, Bearclaws, MudSharks or Mud hogs.
I have given some thought to plumbing it it with a separate shut off so I dont have to remove it except to fill it. That might fool a couple of my buddies when I never have to go on reserve.