Glad you're enjoying the puzzles! -- #8 is nearly ready to go but I'm just very busy this week. Will try to have it up sometime tomorrow... Good luck! Yeah, if you've been driven nuts just go ahead & ghuess, but there is a logical path without trial & error to solve all the puzzles. I've posted some partial walkthroughs to get people started for #4, #6, & #7 (the hardest ones). If you poke around the comments thread you should find walkthroughs & tips from some other solvers too. If you want to be amazed, try the #6-alt puzzle--I couldn't solve it myself (it's a "rogue puzzle" I created partially by accident) but udosuk managed to crack it, & his walkthrough is extremely illuminating if you want to pick up tips on how to approach really, really hard puzzles.
If you want hard ones the other place to go is JC Godart's site:
http://jcbonsai.free.fr/sudoku/There's also the Killer Sudoku Online site, it's linked to on the sidebar of my blog page. They seem to post a rather odd mix of pretty-easy & incredibly difficult puzzles.