While I assume a person could make an argument that MORE suspension travel/flex is always better, I would think that for real world situations, there's a point of diminishing returns.

So that makes my question: "How much flex is necessary/ideal?" At what point does an XJ have enough suspension travel to be a very good "go anywhere (within reason)" XJ?

Correct me if I'm wrong, but it also seems flex in the front is more important than rear (many people live happily with leaf springs in the rear but go long arm in front).

I have 13" travel in front and the same in the rear. But you sacrifice cornering with that much. But sure rides nice and works well off road. The average lift kit has leaves that are too stiff in the rear to flex much. Makes the vehicle retain good cornering and is good for liability reasons

It depends on what you do and if you have lockers or not. Flex isn't as important if you have lockers because if you lift a tire you can still move.

Well, that is a good question. I never measured it. I have about 50% compression with the weight on the ground front and rear. Rear is bastard pack with Isuzu leaves with bottom leaf inverted, shackles 4" longer than stock. Here are some vids of it

But in turn... I could have got to the same place by driving around all the big rocks in my stocker. So it is a good question... How much do you really need?

What do you intend to do with your Jeep is the follow up question.

My little 4" lift has about 12" travel up front and 10" in the rear. I have yet to find something it wont climb. Having a locker in the rear really helps out. A front locker would help even more but I havent found the need for one yet. Just got to choose the right path.

That is a good question. The average guy, not that much. I live in an area where the rocks are big and getting bigger due to erosion on trails. In the Rubicon/Fordyce area, anything that gets used ends up being bedrock unless it is maintained. Both those roads were accessible with two wheel drive cars at one time.

What rear leaf springs/shackles/etc. are you running?

RE 3.5 leafs with one leaf removed. Core 4x4 adjustable shackles. HD offroad no lift shackle relocation boxes.