Kong is better for the chat next to the screen, wastes some time with easier waves. And yes, the game is very much worthwhile without paying, the free section of it is just the same but with less chance for more points. Finally, no content is cut for the first time. I recommend you actually play it before posting a topic.
@thevillageidiot: they were talking about challenges not journey amulets. the 90th wave is achievable but the 100th wave is map specific and none of the challenges appear on a boss map.