Playing them now, and so far, they seem pretty good! I'm playing in alphabetical order by filename (not by level name).
Son of a Birch was fairly simple; I don't really know that Frenzy adds much to the level though.
Tip of the Blade is definitely a very clever puzzle!
I'm currently stuck on "Special Beam Cannon". I've managed to save 88%, but that isn't quite enough... EDIT: Got it. I just needed to swap two skills around. Great level!
Clockwork Grey, I'm pretty sure I backrouted. I saved one lemming more than the requirement, and the Cloner served no purpose in my solution apart from getting that one extra lemming.
Coarse, Grain etc, not a bad level. The time limit is a bit tight, and doesn't seem to serve much purpose, though.
Up a creek without a Basher, this one I'm almost certian is a backroute, unless most of the level just serves as a distraction. I'd also say this is another one where a time limit is unnessecary.
Harder Better Faster Stronger, while visually it's very impressive, if my solution is the intended one it's pretty much just a pixel-precise miner followed by a fairly obvious remaining solution. It could be a good early(ish) level if the miner didn't need to be so precise. But this might be a backroute, too.
Try To Keep Him Digging, you've overlooked a MAJOR backroute here.
No Brakes On The Lemming Train, I think this may be a backroute too, but even if not, it's a pretty good level.
Into The Builderless is another one with a major backroute.
Lemming No Climb Stairs is a solid medium-difficulty level.
Quarantine is another one I'm fairly sure I've backrouted. Didn't need to go anywhere near the zombies, and had plenty of skills left over.
Quick Pyramid is a very interesting concept for a Frenzy level. I like it!
Mobi Did You Shrink The Levels?, this is fairly nicely done. The time limit actually works well here, and while I had skills spare, I get the feeling this is meant to happen. However, in case you weren't aware, this concept is originally Insane Steve's idea (as far as I know), not Mobius's.
Slap Me Lemming Sideways is a very nice level, though the exit's position is very unforgiving. I'd suggest moving it up a few pixels; currently you have to build right at the edge of the platform and stretch the builder almost perfectly at each shrug in order to reach it.