In PvP, if you have imp slam you just have to stop for 0.5 seconds to use it. Generally, whatever you are targetting will not get out of range unless it is already almost there when you start. Also, if you have a stun on the target, there's a 67% chance you will be able to sneak in 1 extra slam with imp slam. Not to mention it's much harder for them to interrupt your cast when it's 0.5 seconds instead of 1.5 seconds. Oh, and if you're partnered with a pally or a rogue you can do crazy amounts of damage in a huge burst if you build up rage before your friend stuns/stunlocks them.
Also, for PvE (and some PvE), imp slam makes it possible to nearly double your attack speed if you go auto-slam...auto-slam...auto-slam. Just for the sake of numbers I've run already I'll use a 39 twink model, lets use 300 damage on a 4.00 attack speed. Without imp slam, you would increase to 632 damage on a 5.5 attack speed, where with it you could get 632 damage on a 4.5 attack speed. 114.9 DPS on one, 140.4 DPS on the other (22.2% DPS increase). I'll say for 2 talent points to increase the potential of an already-potent ability by 11.1% each...it's worth it. Although if you just build up rage and then spam it, it may not be worth it to you.
Edited, Jul 31st 2007 2:53pm by skribs