I just downloaded the latest EAP build from IntelliJ of IDEA 4.1 (build 2002)—4.0.2 is the current stable release—and am quite pleased with it. My subjective impression is that it is rather faster than 4.0 for my most common task: opening classes and files by name (C-n and S-C-n, respectively) and browsing classes from within source (C-Mouse-1). It seems to be about the same with memory usage for a largish project I work on most times. So far, I'm quite pleased and am looking forward to trying out the new JDK 1.5 support features on Wisk.
(See §B.5 of the GNU Emacs Manual for an explanation of notation for key bindings.)
UPDATE: I am having trouble with C-Space not working when there is a declaration of an as yet unimported class. Back to 4.0.2 for a while.
UPDATE: It turns out that C-Space does work, it just doesn't work the same as it used to. You need to now move the mouse over the type needing import, and 4.0.2 is the same. It used to work if the mouse was simply nearby.