It does get old since virtually nothing changes from map to map and it starts getting really hard to get the premium currencies for upgrades later on, but it is still a ton a fun and worth the few dollars needed to buy it.
The person complaining about monkey money is expecting easy wins and isnt even trying to learn how to play the game. Ive only had the game for about a week and my best is 120 on hard. Didnt spend any monkey money or special agents and the only reason I didnt go farther is because I got bored and wanted to play on a new map. This would be my main complaint actually. Once you beat the required wave for that difficulty (85 on hard), there is literally no reason to keep going.
You just need to take the time to figure out the maps, towers, upgrades, and abilities. And you wont get very far at all unless you take the little bit of time needed to level everything up.
KillerSnake616 about Bloons TD 5, v3.9