Entertaining, like most of Crichton's work, with fascinating technology for time travel, and no constraints about time travelers changing the past. While the story was enjoyable, though, I was surprised that the parts of the book set in the past were the least engaging for me. Too detailed perhaps, and more about political maneuvering instead of people.
Awesome! I really like the sci-fi that feels so real.
Timeline is a solid effort from Chrichton, so if you like his stuff, you will probably enjoy it. He works the time travel angle in a novel way, which is nice and it gives us a great excuse to learn lots of interesting things about the middle ages. The action feels a little "action-movie"-ish, its fast paced, shallow, and sometimes doesn't make that much sense. The characters are about two inches deep. I can complain too much, though. I read the book in three sittings and had a good time with it.
On a side note, the research that he did (or that assistants did) for this book must have been staggering in scale, even supposing that he made a lot of this stuff up.