i built a program that solves these things – I put a level 5 puzzle in it and it solved in about 1.5 sec (normal), and i also put a level 8 in it and it locked up – it uses the same processes that i do when i do a puzzle (i modeled the logic after the logic i use to solve a sudoku) and it found that the puzzle was not possible to solve

so yeah, level 5 was great (i went back and did it myself and found it quite challenging) but some of the higher levels need some definite work – instead of reducing the amount of numbers that populate the puzzle, perhaps you should experiment with the combinations of numbers
for example, instead of having a puzzle that starts out with having, say, 5 1’s, 4 2’s, 1 3, 5 4’s, etc, have it start out with like a max of 3 for any one number – that would keep the puzzle in the realm of possibility, but drastically increase the challenge of it

all in all, i’m quite impressed – good job
if you want some more feedback, tips, whatever, just email me (make sure you put sudoku in the subject so i’ll know who you are and don’t delete it)

hope i helped!