Generally here's how it works:
- You'll appreciate the hell out of it whenever you just want internet access and are not at home. No crappy wifi portals and passwords to deal with; you get to (a) stop wasting time and (b) just drink your coffee and browse the web as if you were at home. Or maybe you're out travelling and want to use a map app or something - 3G/LTE solves a lot of problems quickly.
- If you have a phone with tethering, it's less helpful (you could just connect to your phone and get the same thing for free) but you may appreciate it more at times simply because you don't have to kill your phone battery to do it.
- You'll hate spending the extra money if you find yourself only using the internet (or your iPad in general) at home. In that situation, it does nothing for you and you'll probably drop your LTE sub in a month at most.
All in all, I found cellular support useful, but I stopped buying it on my iPad's after second gen. YMMV