1.Did you manage to complete the game?
No
2.If no, which level did you reach?
11
3.Keyboard Layout - Alternate suggestions for controls?
I was find using up/down
4.Is the learning curve steady enough?
I had no deaths until reaching level 4, then couldn't make it past the first jump. I was able to get that far by jumping both guys at the same time right at the edge of the platform. I may not be coordinated enough to manage two guys, but it felt like a big difficulty spike to me.
(After playing on low speed mode I realized you could just let one guy drop and only need to jump for one of them. If you want to smooth this perceived difficulty spike out you could add ceilings there to make it more obvious that one runner shouldn't be jumping.)
5.Did you find any bugs or inconsistencies in gameplay?
No
6.Were the characters clear enough?
Yes
7.Was the moon/sun distracting at all?
No
8.How did the gameplay change when a second player was introduced?
Sorry, I don't have anyone else around so I can't comment on this.
9.Did you try the different speed settings?
Yes, but only after reading this question.
It helped me see what to do on level 4.
10.Did your characters disappear off the screen and not come back at any point?
No
11.What was your favorite level and why?
Level 10, I bet it looks really cool with two players playing.
12.What was your least favorite level and why?
4 because of the perceived difficulty spike.
Also, if you haven't seen a Bubble Bobble speedrun I highly recommend. This game reminded me of how coordinated the players are.
http://speeddemosarchive.com/BubbleBobble.html