My wife let me buy the XtremeMusic version last week. Can we say "saaaawwwweeeeeetttt!"?
I'm coming from a SoundBlaster Live! 5.1 card (the onboard on my MSI sucks... I miss nForce2's SoundStorm!), and the difference is night and day. Everything's clearer, crisper, and generally better sounding.
This particular part has three modes: entertainment, gaming, and recording. The entertainment setting is used for music and movies. If you know anything about music, you'll know .mp3s are two channel music files (left and right). Yeah, I know about multi-channel .mp3s, but your average .mp3 file is going to be stereo.
I've given you this information because this is where the X-Fi shines: my stereo .mp3s are being played as multi-channel. The processing unit pipes the music to whichever speaker it thinks serves the best purpose. Most vocals come out the center, while the main music comes out the front left and right and the background comes out the rear speakers. I first noticed this when I plugged in headphones and left the speaker setup as 5.1. This functionality alone is worth the $140 upgrade.
Oh, but the value doesn't stop there. The main reason I wanted the X-Fi was Battlefield 2. There's an Ultra-High audio setting only available for X-Fi boards. Uh... WOW!!! It sounds friggin awesome. One note to consider: you must use the gaming mode in order for BF2 to pick up that you have an X-Fi. I wish I would've known that before I uninstalled and reinstalled drivers (doh!).
The bottom line: If you don't want to spend the $400 for the Elite Pro, the XtremeMusic is awesome and well worth the cost.