I have the Audigy 2 platinum (pre-ZS) I really like the Live Drive thingy because of the availability of the many front inputs for headphones and mic in games plus optical outs, midi, firewire port for my iPod etc. (I never cared for the remote control though and never even used it)
I also play Electric Guitar so I plug my guitar into my Zoom FX processor and the FX processor out into the front line-in of the Live Drive and have the sound come out of my Logitech Z560 and can directly record what I play on the computer that way with great guitar effects. The front drive is just very convenient for what I do, before I had the Audigy2 I used to have a NewQ Platinum for mic and headphone front inputs only, that worked for 2-channel strereo doubled to the rear speakers only and couldn't work right with audigy 2 you lose some functionality, FrontX is the same you lose 5.1 sound if you use it with Audigy2. Live drive is the most useful and convenient part of Audigy2 package for me at least.
Oh and btw, if you already have a 4.1 system and don't have the cash to buy a 5.1 yet for doom3... You can always do like I did. I bought a 2.0 labtec speaker set just to add a center channel to my current logitech 4.1. You plug it in the center channel plug in the back of Audigy2 card, now, only one of the the 2 speakers will work but that's fine since center channel is MONO anyway, you then set the speakers sound balance ONCE and then after that initial set-up just adjust the sound level of all 5 speakers via your keyboard volume knob or windows volume. The speaker that is not working from the 2.0 set is hidden under my desk out of the way, the center channel sits on top of my 21" monitor. This works great in DVDs for now until I'm ready and have the cash to get a proper 5.1 but this works in a pinch and I don't see any reason to change my great sounding Logitech 4.1 set yet just for DVDs and a few odd games. I mostly into music anyway.