Pretty good until you get towards the end
First 3 worlds: Fun, exciting, combo thing is great. Good times, if a tad easy.
Last world: Disastrous. The map is gigantic, meaning you can't get a combo, and enemies are too far away to be any danger. Most enemies pose almost no threat at all, but just take forever to kill. Great design there. The last weapon, the energy thing, blows dick: upgraded or not, and it doesn't just cost a ton to buy, it costs a ton to upgrade too. The very last enemy moves at a snail's pace (literally), shoots once every 15 seconds, and the shots move only slightly faster than he does. Of course he takes forever to kill (yay), but luckily you can run him over to take a whopping 40 dmg. Yay! I feel accomplished!
Overall progression of gameplay: start, fun, fun, fun, FUCKING FAIL OMFG THIS IS FUCKING PATHETIC, done.