It's nice how many puzzle games can take a simple concept and stretch it to its limit. I finally beat this after about an hour, and I thought the tricks on some of the puzzles were pretty neat (the last one in particular was fun to gradually work out).
As other people have mentioned, having two different tools to place/delete blocks was somewhat cumbersome. Some people suggested being able to click on blocks to remove them, an alternative would be to have a hotkey (e.g. ctrl+click) to remove blocks. You can press 'E' to switch between them, but I personally find that controls which start when you press a button and stop when you release the button are easier to use than hard toggles. It also makes it easier to remember which tool is equipped - maybe this is just me, but when I see the circle with the pencil in it, I assume I have the eraser out and need to press the button to GET the pencil (and vice versa), so I kept getting momentarily confused about which tool I had out. Most of the programs I use in my daily life work that way, so it's a hard habit to un-learn.
Anyway, fun puzzles.