I'm still playing the game, but I thought I'd jot a few notes down before I forget. First, it's so refreshing to play a hidden object game where it takes some time and effort to find the objects -- that's the most enjoyable thing about this game for me, aside from the post-apocalyptic scenery, which is totally cool. I don't really understand the story-line (assuming there's something beyond a guy wandering through weird scenes), nor do I know anything about the main character besides the fact that he seems to know where he's going; we don't get any real dialogue or emotional responses, and thus far there are no other characters. I'm also not sure what the motivation would be for you to redo scenes to get more points. What are the points worth? Maybe it will become clearer later in the game.
But if you are just looking for a nice time finding hidden objects, this will certainly fill your time. Also, it looks like the in-app purchases are just hints (since the objects are genuinely hard to find, maybe you would get frustrated after a while waiting for the hint button to recharge?), but I haven't spent any money at all.