My boyfriend and I took a zip-line tour in August 2012 with Vertical Illusions. We decided to book with them after reading amazing reviews on Trip Advisor, and we were not disappointed! I'm a little weary of heights but NEVER felt scared or ill at ease. Our group was small (4 of us, plus 2 guides) which meant there was not a lot of wait time between zips. We were all first time zippers, and the guides were amazing at teaching us the technique while still making sure we thoroughly enjoyed the experience. You get to zip 15 times on this tour! It's a hefty climb to the top, but again, the guides will stop whenever you need a break. They even have stops planned throughout the climb for photo ops, and just to make sure everyone is ready to continue the climb.
I can't say enough good things about this experience. If you are doubting whether to book a tour or not, DO IT! You will not find more dedicated, knowledgeable guides, or a better experience anywhere in the Dells!