Not very accurate. Audio Technica M50 phones are "studio monitor headphones" and offer an excellent amount of great quality bass. They're probably the most recommended headphones for bass-lovers, and over at Head-Fi they're some of the most recommended phones period.
EDIT: For the record, people really need to stop being so extreme. The Sennheiser certainly start dropping in the lower frequencies, but saying they have no bass? I'm sorry, but that's terribly inaccurate. They're not suited for dubstep/techno, but for classic/hard rock they have a very even and accurate bass. Even then, nothing stops you from EQing the bottom frequencies as much as you desire. They still won't rattle your skull, but you can get a very impactful punch.
I just felt this important to point out, because when potential buyers see that a pair of nice headphones have "no bass" they can take that literally.
If they have substantial, inherent, un-eq'd bass, then they are not true "studio monitors." The description sounds more like DJ headphones. I can't comment because I've never heard them personally.
The only way to add more "bass" to Sennheisers is to ramp up the db output, or equalize them. In my personal book, this doesn't count, and sounds weak and artificial; all you're noticing are the drivers hitting the lows harder because the volume or levels increase on the lower Hz. Just my opinion however.