Hey! I worked safaris!!
Yes, you only get a week to learn it. But I started memorizing it the first day. The safari (and actually, the other ones) isn't that bad because you can separate it into smaller chunks. IE: entering the forest spiel, rhino spiel, okapi spiel, etc. and then you just interact with Wilson. It's really not too bad, and gets much easier when you're actually driving.
I used to be nervous about getting on a microphone and getting people's attention: not anymore!
Anyways, I loved it. I told them in my interview that I wanted a position around animals (thinking I'd get maybe a walking trail), and they asked if I had spieled before. I had, nowhere near the scope of the safari (just safety spiels at the local amusement park), and they kept asking if I was comfortable with long spiels. Most of my co-CPs were theatre majors, as well.
It was great to me because you got to always be doing something, and could mix it up a bit. It is pretty intense, but I thought it was way more fulfilling than standing all day pushing a button or checking lap bars.
If you have any more questions, I'd love to answer them!!
ETA: One big difference between safaris and most other spiel roles is that the safari lets you get off-script a little, mostly because we never know what order the animals will be in or what they are doing. GMR especially but most other spiels have to be spot-on, no matter how silly you think you sound, which I think would make it harder to do and more repetetive.