Flickr users are definitely not happy with the change. As far as I can tell the response has been 100% negative. Aviary is very basic and Flickr did way oversell it. They also told their userbase that Picnik would be available until the 19th of April. This is not the case.
Whatever the merit of the software the roll out of Aviary has been very badly handled by Yahoo/Flickr. There was a general acceptance that this was not initially a problem of their making as there wasn't much they could do when Google decided to fold Picnik. Saying that, with the appearance of Google Plus the foresight of Nostradamus was not needed when working out how this woulf all pan out.