We got Thaddius tonight! Kael'Thas seems to have very solid hardware; we had literally no server lag issues playing at prime time. We encountered none of the lag nightmares that some guilds have faced that literally made him impossible without learning the encounter at 1:00 AM.
Of course even without server side issues, client side problems are still pretty prevalent. A handful of our members would randomly disconnect when we engaged Thaddius almost every attempt. Even more would disconnect infrequently, but still enough to put a serious damper on things given the nature of the fight. It really stresses individual awareness and movement, such that inopportune disconnects have a chance of wiping the raid or at least killing a handful of people. On the plus side you can get away with a handful of deaths and still kill Thaddius before he enrages. In our killing attempt we had two early deaths and still managed to take him down with 30 seconds to spare.
The most disappointing thing about Thaddius though is the first phase of the fight. You have to defeat two pretty buggy and strange minibosses before engaging Thaddius. They're pretty easy, however killing them with no deaths can end up being a problem due to some very buggy threat issues. We also found that if one of your tanks is just a bit laggy they become nearly impossible..
Ultimately though the fight is very fun, probably one of the neatest designs in Naxx, it just really needs some bug fixes.