This is a very strange situation. Some of the servers are clearly broken and Bliz doesn't know how to fix them. Others, including some very high pop ones, run very well and have little downtime.
As to lag, again its very strange. Sometimes I'm in a truly congested area and everything runs smoothly. Other times I'm in a barren area and lagging like crazy.
I think I've said this before, but I'm really curious to learn how WoW works on the inside, how the servers do instances, the seamless world, and why Bliz can change some things very easily and other things need full-blown patches.