First, I have to mention that I've read the visual novel last week-end and it totally blew me away, I just loved the complexity of the story and thought that Arcueid was quite a genuine character. So yeah my expectations were quite high. I'd say 8 out of 12 episodes seemed to be written with almost the same script as the game, I really loved those because I could see animated one of the stories I've enjoyed the most in any form.
The other 3-4 episodes were boring at times though, since it was relating to very "minor" scenes, or other routes of the game that didn't bring much to the main storyline. Some of these scenes were totally new to me; a complete episode in an amusement park... the hell was that? Maybe I missed a chapter in the game or something?
So overall, I really really liked most of the series, but I also disliked the scenes that weren't based on something I've read in the visual novel.
The artwork isn't stellar, but it's still pretty good for an anime released back in 2003, at least when compared to some others I've seen. But one of the stronger elements of Tsukihime is definitely its soundtrack. I'll never get tired of watching the OP over and over again. Plus, the background musics during the show is just amazing.
Still, I can understand why Tsukihime isn't in many top 10s, (hell, it's 377th on ANN... thats a pretty bad score). Well first, 12 episodes is way too short. But also, as an adaptation, the anime was produced under the assumption that the viewer has played/read the game. So the story obviously seems incoherent at times to those who haven't. That might be one of the only weak point though.
One thing's for sure, the anime is in my opinion an awesome adaptation of the game. So my advice to anyone who likes dark toned stories like this one, get the visual novel first, read it all and then watch this. You won't regret it.