Yes - after I got bored of regular Sudoku, I found "Killer Sudoku" and I've been playing that variant regularly for a long time. The hard puzzles (blank with no "easy" parts) in the killer variant are quite challenging, and over a span of years I've continued to gain new insights into better techniques for finding the answers. The logical chains you have to build up just to eliminate one possible number somewhere really stretch the brain! More often they're merely tough and I can solve them in one sitting in under half an hour. It's a great mental warmup in the morning with coffee, to get the brain juices going!