the last level was way to tedious. i couldn't figure out how to fight the impending horde, so i just stockpiled until i had enough resources to make all the toxin at once. the mission timer was at 30:00 when i won. still pretty fun though. but it had more tutorial levels than full freedom so just 4/5
the checkpoints are a problem, yes, but they wouldn't be if for one detail. the relative slowness of your plane and inability to fly straight up ruin everything. yes, stalling is realistic, but its a flash game! not a flight simulator!