I have the 70-200 IS 2 and I can the IS is really nice to have, especially if you use the 2x extender with it.
I've used it for a fair bit of surfing stuff and when the wind is blowing, the lens has constant micro jitters when the IS is not on. When the IS turns on, the micro jitters are completely gone.
"It depends on what you're shooting".
I don't own a single lens with IS and can't imagine why you'd ever need it, BUT I also only shoot narrative. If I did a ton of handheld or doc work I might think it was the greatest thing ever.
If you live on camera support all the time, it's not so necessary. BUT, you can always just turn it off. You'll never regret having the option to use it.
+ 2 for IS. IS is extremely important for DSLR work. I can't even handheld 50mm properly, let alone at 70-200mm. IS is - for me - necessary. I hate the DSLR microjitters that appear when handheld.
The 70-200 IS is an awesome lens. Be aware that the IS uses a lot of battery. You can feel it making micro adjustments all the time. Still, couldn't live without mine.
CK
IS is important for handheld/stealth (no rig) as well as tripod when at the long end (~200mm): it stops micro-vibrations that may be present (which are challenging with rolling shutter).