It's been a few years since I last did serious javascript development. The tools have certainly improved a lot.
Having performance issues somewhere in several thousand lines of javascript is never a good thing, but with a little data from FireBug trying to find and fix the problem isn't nearly as scary.
I'm also really liking jquery - most of the stuff it does is no big deal if you are used to working on the server, but if you're used to writing large amounts of javascript with no framework even being able to find elements by class is pretty amazing.