As has been mentioned already, they can be useful, but they can be dangerous and costly too. If you get one then learn to use it. People have smashed fingers and hands from them "kicking" back. One incident was posted here years ago which I can't find now showed a guys thumb split open and about twice the size as it should have been when it was smashed between the handle and the body of the jack. They can also slip and smash doors or other parts of your vehicle too. People on here in some of the threads have mounted them on their roof racks, some along the bottom of the front of the back seat, and some on rear bumpers or swing outs that were set up to hold one, some also have them mounted on the bar on their ARB front bumper or it can be mounted on the bar of a similar style too.
Plenty of accessories out there for them to use depending on the job being done. Base plates for mud/sand, hooks to lift a wheel, adapters to let it be used on sliders, etc. Make sure you have it setup so that it doesn't slide such as when on a rock base or say pavement. Put something like a rubber mat so that it can grip and not slide out. Maintenance is something that is overlooked too. Being mounted out in the weather can mean that water, sand, dust etc. can get on it and gum it up and if steel or part steel cause rust so keeping it clean and lubed so that it works and lasts is something that's needed. There are covers for them or sacks too, but even then you need to check them as the jack can rust if any moisture gets inside the bag. The last thing you need to have a situation where you need the jack and it won't work because it's rusted and gummed up. Just some things to think about.
I've used them with my Dad many years ago and with friends too. Dad moved mobile homes as a second job and we would use them to slide the tongue of the home sideways to gain access or get it into place when parking it if the truck wouldn't fit. We used it on our backhoe too. My friend used it on his property to stretch fence, move sheds, etc. A great tool when used and respected.
I rushed out and bought one back in about 1990-95 sometime as I was starting to take my new pickup off road and, well everyone had one. I've still got that jack and it looks just like it did the day I received it from UPS. That's because it's sat in the corner of my living room ever since then. I've never used it once. I haven't needed it in all the trips on back roads or to Moab that I've done over the years. I've been strapped out of stuff and my stock jack with a couple of foot long 2x6's has worked good enough to change tires with. It's probably going to be worth a fortune sometime as an antique and worth even more since it's in perfect condition. My relatives might make out pretty good on that.
Here's a few threads on the Hi Lift. There are probably more under different terms such as mounting or something too.
Toyota FJ Cruiser Forum - Search Results for hi lift jacks usage fj cruiser