I’ve played this a lot today. Which is evidenced by the fact I’ve completed the final four levels (and hence the game), getting a gold on each, and each taking (on average) an hour. Yes, so I played it a lot.
It started to get really tricky near the end, especially as it threw loads of tanks and turrets and inky walls and bottomless drops and all sorts at you. The final level, on a space ship, also had a boss – something there hasn’t really been previously in the game.
So it’s now complete, albeit missing the bonus missions I’ve unlocked along the way. Those I have completed there, though, have been much shorter than main levels however – around 5 minutes or less each – which is good for going back to and dipping in, I suppose.
Final thoughts? It’s an excellent game. It’s quite long, doesn’t get boring like you might expect it to (after all, you do little other than colour stuff), appeals to people like me with Gamer’s OCD, and has fantastic jazz funk music. My only minor complaint is lack of mid-level save points, which is a pain when the level might take 90 minutes to finish. but, as I said, a minor complaint.