We did the zipline tour at the end of a 2-week tour of Alaska. We have two teenage kids and I wanted to finish with a bang, so I reserved in advance, agreeing to pay $600 for a family of four. I thought the kids might look back and think it was one of the best things we did. Instead, when mixed in with all the other fun things we did in Alaska, we thought the zipline tour was ho-hum, and even a bit boring.
The whole thing takes about 3 hours and you literally spend a total of about 3 minutes actually running a zipline (if I recall right, there were 9 runs). They ask you to arrive 30 minutes in advance, which was unnecessary, and then they have to take you to and from the place in a van. We had a full group of 8 and the outfitting and safety briefings took a long time. They are so particular that you follow their procedures exactly for staying straight, keeping your feet together and stopping with a gloved hand exactly how and when they say that we felt that distracted from enjoying the ride. One poor fellow had trouble following the rules, but not in an unsafe way, and one of the two gals was getting on his case about it beyond the call of duty.
The bottom line is that it might be a fun thing to do if you are on a cruise and take a day trip to Talkeetna. However, don't expect to compare well with many of the other fun things you can do in Alaska.