I rarely made it any further than level five, and when I did, it was usually because of a nick-of-time power-up rather than skill. Power-ups can swing momentum to your favor really quickly, though with the exception of Score Multipliers and Grenades, they're rare, unless purchased between games. Coins are earned by playing, and you can usually earn a decent number of coins (about 10) per game after a few games. Though if you want a constant stream of power-ups in each game, you need to purchase them via the in-game store for real money.
The system is similar to what is used in most Facebook games, especially Bejeweled Blitz. Coin packs range between $0.99 and $9.99, which isn't a terrible deal if you're that determined to see past the first few levels. At the same time, I'm still not completely comfortable with tying progression to monetary values. At least power-ups are relatively cheap, so coins can last a long time if you ration them out. I also liked that coins are "gifted" to you every few days, so even though I probably won't purchase coins, I still have access to power-ups.
Actually, the ability to "buy my way" through the game was a big motivator. I think I played more games just to prove I could win without purchasing power-ups than I would have if they were easily available. It also encouraged me to figure out my own little tricks for easy points, such as tapping Mooniz as the pile fell into place before the timer started.
As much as I didn't like the in-game micro-transactions, I still enjoyed Mooniz enough to play it every day for a couple of minutes since I received it. It was even one of the first games I installed on my new iPad, so I'm sure I'll play it a lot more in the future. It's a challenging game, but Mooniz is fun in quick, fun bursts.