A minute ago I loaded up the test server with a fresh copy of the MazzleUI, version 1.01. This had no alterations to any of the addons in the package, and no additional addons installed beyond what comes with the normal MazzleUI package.
Honestly? It wasn't nearly as bad as I thought it would be. I immediately started getting a few errors just at the initial loading screen after installing the addon files, which gave me a bad feeling. Fortunately though I was pleasantly surprised to find that nearly everything mazzified the first time without any real problems.
At first glance the only thing I noticed that wasn't mazzifying correctly on a level 1 priest was the autobar. The mazzifier had no effect on it, and it was left as a green dot in the center of the screen instead of being moved to the proper place and having the buttons setup. I wouldn't exactly call this an in-depth test, but so far I've only found a few minor errors being thrown at me during combat and while looting. So far I've gotten about 11 errors from playing for a few minutes, and it doesen't throw an error every time I fight or loot. Either way, the errors weren't preventing me from completing quests as I normally would.
So far I'm pleased to say that the UI is very playable and hopefully should only require a few tweaks to eliminate the errors being given. Keep in mind though that this is only based off of a few minutes of testing a level 1 priest, and more serious errors could present themselves with a level 70 character of a different class.