Yeah, knew it was THX certified. I've seen a couple of people moaning (mostly about their opinion that the system is unable to deliver enough bass), but i've found it to be pretty excellent (bass included) with CD and my iPod. I think the problem most/all of the time is that you have to take a lot more time than usual to get the setup right for the source - you're given 3 memory settings on the EQ because you need to use 'em i've found. My settings for CD and iPod are definately different, for example.
As i've said, you have to get the right iPod EQ setting if you're using one, then the right treble/bass settings on the car's stereo (the ones accessed by the Tone button - they're universal to all sources, so they have to be a nice starting point for all your inputs: iPod/CD/radio - I tend to up the bass there by 1 or 2 steps to give a nice punchyness that can then be exploited in the next step), then finally setup the 3 memory settings in the 7-band EQ for each source (where you can further boost the bass at the lower frequencies, or reduce it by lowering the lower frequencies. Given that I don't listen to the radio, I use 1 each for CD, iPod via Intravee II, and iPod via the aux-in socket (when I have a passenger, it's easier to give them the iPod to choose music, rather than via the car's controls, and the iPod-via aux-in seems to have different audio qualities to the iPod via Intravee II).
The only real downsides to the system i've found are that you can eventually overdo the lower frequencies, where the subs behind the seats can start to make a Rolf Harris-esque flopping/popping sound. It's the downside to their relatively small diameter - they're very punchy and accurate, but you can overdrive them using the EQ. I tend to find that 3/4 up the lower frequency bars is about the nice spot for powerful bass without the popping. And the second is that the 'good bass at low volumes' claim by BMW is false - they seem to suffer just as much as any other car's system where the bass is the first thing to be drowned out by road noise when the stereo volume is fairly low.
Overall, though, it's an excellent system i'd say.