Frustrating but addictive and fun.
This is the only game I've been playing for three days straight, of course with breaks. I find it challenging at times, and also frustrating, but fun.
The challenge comes in thinking how to tackle a scenario, the frustration comes in with often annoying jumping abilities, but the fun comes in as you progress and remember what strategies work for enemies and portals.
I love the art style and think that it's one of the advantages for this game, plus everything runs so smoothly so you don't have to worry about lag. However when playing in 'pop-up' sometimes I'll re-spawn at a point I haven't activated yet and so two Blue's spawn but I can only control one. It's a minor quirk which is solved by closing the window, though.
Controlling blue is simple, but jumping from platform to platform can often be too much for him/her/it to handle. Often a minor run up solves this but when there are narrow platforms and long gaps it can add to frustration. Having said that I don't want it to be easy, so I can let this slide as I like a challenge.
Difficulty per scenario varies and I suppose that keeps you on your toes, and reminds me of Left 4 Dead because that game toyed with you, unleashing a horde when you least suspect it. I find your methods interesting. and somewhat similar. Each scenario is different in regards to difficulty.
I like the 'plot' of this game too, as it cuts away the need for power-ups or for talkign between characters and presents a wonderful game withouth the clutter. You can sit down, play the game and enjoy it, not to mention it saves your progress (major plus!).
Generally speaking I have no qualms with this game as it has many strengths, but few weaknesses. All I can end on is well done on an excellent game!