I really loved this game and am happy there are more levels to play. But I understand a lot of people's frustration. Though I think going and leaving 1 star reviews just out of spite is pretty childish and the game should be judged on it's own merits rather than a decision to charge for DLC. A lot of reviews mentioned how they hoped there would be more levels added in an update. Being that you have to pay for the new levels is probably surprising to a lot of people who were expecting a free update all along due to how short the main game is and they wanted more value for their $4.
I personally don't mind paying the extra $2 for 8 more levels. It's not that big of a deal. The developers put extra time and care into crafting more levels for this beautiful game, they should get a little compensation for it. The way I look at it, $4 for the main game, which lasted me an hour to complete but I've since completed it several times more, is the price of a burger at a fast food joint which I eat in about 5 minutes and is forgotten about 10 minutes later. Here I enjoyed a wonderful little gem of a game that has long lasting memories of my experience with it. And $2 for 8 more levels? I tip $2 to the pretty cocktail waitresses every time I get a drink in Las Vegas, where I live. So yeah, not a big deal.