If you're staying at Treasure Island, admission is free (a $34.00 value). To me, that's the only way to go. I came in on Christmas Eve and it was pretty busy but they get you checked in and on your way in a timely manner. You can download an app on your phone for the complete interactive experience or you can rent a device for $5.00. I rented the device as I didn't want to download a big app and not have my phone camera available for pictures. There is also a $5.00 service fee that applies...so for $10.00, I was set to go.
You go through this experience in a big group but you can go at your own pace once inside. Basically, it's just some neat props and tons of information about each of the characters of the Avengers team. The rented device/app gives additional information about the characters and the opportunity to take a quiz and earn points,etc. It's visually really cool, especially Thor's area.
It's definitely something to do if you're a hotel guest at TI. I spent about an hour and a half there because I was wandering through the retail store afterwards. I had a good time but I can see why some people were disappointed. Even if you don't want to go through the "experience" part, the retail store alone is something to see.