I'm torn on this: there's a lot of good things in the level design -and as h4724 says, the bonus map and end map are well conceptualised (although I also thought the design of map 4 was good as well, on that level). I really liked the ideas in most of the maps...
On the other hand, this remains super hard on skill 0, to the extent that I had to save almost every 30 seconds on some maps to make progress, and I eventually godmoded the final bits of the end map after about 15 attempts (just on the final section - the final rune, teleporter sequence and cleaning up).
This also kinda stopped me from looking for secrets as much - there's quite a bit of jumping in some maps, and I was exhausted enough from not falling off (okay, I did fall off quite a lot) that I didn't have the energy in me to go looking for red boxes to shoot, after map 1. I think I found one secret on map 3 (the fake lavafall that's shootable) but otherwise mostly tapped out.
(As such, I did the bonus map via changelevel...)
Anyway, with some tweaks for skill 0, I think this would get up to Excellent for me...