We've seen
123 Imagine and the Hero one with Grover. I thought the 123 Imagine show was amazing, my kids loved it, my 18mth old at the time stood the entire show and danced and it was very interactive. We bought the CD and still listen to it two years later. The Hero's show wasn't as good. We still really enjoyed it though. They came out into the audience quite a bit. If you pay for seats close to the stage, that's kind of the payoff, being able to see them close and maybe get a hug. If you visit Busch Gardens in Tampa though, you can see them up close there. They have a brief show at Busch.
I'd love to take my kids to 123 Imagine again, Hero's is probably only a one time must see, but if it came around we would go. I'd probably take them up until the age of 6 or so, my niece went with both times, she was that age and loved it still. Also, it's worth buying the 123 Imagine CD to listen to.