I ran nerf bars on my warrior, it let me ride a little more confident, knowing that if i slip i'm not going to run my own foot over and end up with a broken ankle. They did actually save me from slipping off at least once or twice, i think it was in the snow or mud when the pegs were slippery, and i think once i jumped it and got all caddywompus and missed one of the pegs when i came down. I don't have them on my 700 yet, at first i didn't have the money, and now i would just rather spend it on other things. Nerfs are a good idea though, especially for a more aggressive rider. But are nerfs worth it? I would say yes. $150 and a couple extra pounds to potentially save yourself from a hospital visit and months of limping around and being unable to ride? Makes sense to me.
I've never used riding boots, i just wear sneakers or old work boots. That's another thing that I SHOULD have, but haven't gotten yet. On a quad i see no big necessity for them though, especially if you do have nerfs. I've been riding quads for nearly 10 years now and i don't think i once came across a situation that they would have helped me out. I'm more interested about the boots for riding my bike because it doesn't seem like it would be too difficult to mess your foot up if you start losing it and have to catch yourself.
If i were to get boots just for quad riding, i think i would go with the halfs. A little extra mobility could be appreciated when you have to get off and drag it off of some rocks or something like that.