Whether composed by hand (as I suspect), or by an extremely clever piece of software, these puzzles are rather satisfying to solve, with frequent little numeric insights. I've not found them overly difficult - suitable for a leisurely cup of coffee. As grids get larger, of course there's more chance of making an error - I've found the 8x8 grids more prone to this (and more difficult) than the 9x9 for some reason, and it pays not to rush.
On the minus side, the paper they're printed on is not as good as that used by the Times, although it is (just) good enough. The claim to 'make you smarter' also irks - either you enjoy this sort of puzzle or you don't, and that's all there is to it.
I've completed about half of each grid size in the book.