I was lucky enough to run x-crawl with my last group before it folded. Even though it was the most challenging game I ever ran, I don't think I ever had as much fun playing D&D since.
Just to relate a funny story about my x-crawl game. After running the PC's through the crawl in the core book and the Three Rivers Crawl, I was drawing a blank on what to do with this group after that. Then I had a brainstorm! I will shunt them off to Ravenloft during the crawl. Using the When Black Roses Bloom adventure. When the demiplane of dread came and claimed them in the middle of a room, they thought it was all part of the crawl. Oh boy were they wrong! To make a long story short, they ended up coming home in the middle of central park with a vampire kender on the loose. Not to mention they had to explain the the NAE why they disappeared in the middle of a crawl, on international TV. They ended up clearing themselves with the NAE and made a movie based on their adventure (claiming it was a work of fiction). It was a great way to end our run of X-crawl.
