This looks great. I love the shake and splat too; great feedback, make the player's fails feel like fails.
I think the standard way of approaching "freemium" IAP's is to have some goal that's improbably hard but can be circumvented with money. This way you have a tiny hardcore segment who will just grind to get the game in its entirely for free, or more casual players will go "hey, I've got some money on my account, I'll skip". I know I did the latter with Kingdom Rush on iPhone, but if the grinding option hadn't been available I probably would've been less inclined to give the devs any money; seems a bit mean to entirely lock people out of content; a sharp price ramp from free-to-expensive is what I'd call the "drug dealer at a party" business model. Haha. Anyway, I'm sure there are dev blogs out there which have actual figures on instead of one person's opinion.