The general impression we are getting from twitter and IRL is really glowing feedback from everyone that attended CryptopartyLondon3, we smashed it! however were wondering about getting some kind of structured feedback... so please send us tweets with suggestions. But at the end of the day, everyone seemed to enjoy themselves and that makes all the difference.
We will, however, email out a feedback form (the the option to be anonymous) so people can provide feedback in the way they want to. :)