Played all but the last of the Balloon World levels and I would say they're worth it. I think. Hard to say given it's that much more expensive than most other Android DLC, but they're just as good or better than the ones already in the game. The only real downer is my phone (stock Desire on 2.2) simply cannot handle HD graphics turned on for most of them - they're that much bigger and there's more visible of the level at any one time it turns things into a slideshow every other minute. Still, I'm impressed the devs built in the ability to switch from low to high detail and back, and the menu always comes up straight away no matter how badly the game's running, so it's not that big a problem.
I would buy more level packs at a bundle price, but I hope they don't go crazy sticking out a ton of them, and it would be a decent gesture if they kept some of them free. We'll see, I guess.
Partly I feel like I ought to give the devs some money to show them not everyone's like the whining little sods on the Market at the moment. The comments make me ashamed to be an Android owner, quite honestly - putting ads in your game doesn't magically make enough money for everyone, idiots, and the devs did not say the game was always going to be completely free, now and forever. They were saying there were going to be paid levels since ages ago. If you don't want to pay anything, go back to Angry Birds and leave the rest of us in peace.
i kinda figured that would happen. i guess it was to good to keep being free