We've not ever been to TSI in any of our trips to WDW with the kids, and I don't remember going as a kid (despite liking Mark Twain's stories). But, we have been to TSI at DL with the kids... they had a blast, but like half the things to do on the island were closed. Basically we just did the pirate treasure, a few caves, and that was about it. A nice break, and a chance for them to run around in a way we wouldn't let them in the rest of the park, but not something we're dying to squeeze in on our next trip to WDW to get a fuller experience than we got at DL.
New Cars rides, though, sign us up...