Has threefish successfully been attacked (practically or theoretically)?
Full disclosure — I'm a Skein/Threefish co-author. Also, when I mention Skein/Threefish without any other qualification, I mean Skein/Threefish-512.

The security proofs we did for Skein prove that if there's a weakness in Skein, it implies an underlying weakness in its components (Threefish or UBI). As Dmitry says above, Threefish is very strong, and there are very few attacks on it. I don't know of one that's better than the 35-round attack we mention in V1.3 of the paper.

Our goal in Skein was that we know a lot more about designing block ciphers than hash functions so we'd build a strong hash function by starting with a strong cipher. Threefish builds on Twofish, (P)Helix, and other things. The philosophy was lots of fast rounds, and that has held up.

Enough time has passed that I'm starting to use Threefish in some of my own work. I think it's a very good way to go because tweakability is a powerful feature. If you are interested, also look at McOE mode which can turn a cipher into an authenticated function, and is designed to be resistant to misuse. It is built with AES and Threefish as examples, and I think Threefish in McOE mode is powerful.