I've just started playing these levels -- so far, completed the Fun difficulty. Nice designs, appropriately easy, fun to solve. The only thing I'd say is that Fun 26 (The Lemming Tower) has a long solution with a lot that can go wrong, and although every part of the solution is familiar and simple to veteran players, new players would find this inappropriately hard for a Fun level.
Solved Tricky 1-10. A very nice way to steadily ramp up the difficulty, with a mix of tougher "open" levels (lots of skills and various possible solution methods) and puzzles with limited skills. I originally thought "Fortissimo" was way too hard for its place -- then the completion screen came up and I realised I'd gone for the 96% solution and it only required 25%
"The Good, the Bad and the Ugly" had me stumped for a while -- in the end I used the bomber and miner to reach the exit and the builder to delay one lemming.
At the moment, completely stuck on Tricky 11 (Scrambled Lemmings). You can only lose 16, but it seems to take much longer than that to build a safe landing -- my best so far isn't even 50%.