I've heard them before, they have shitty mids and highs. But lets talk about the bass:

While the lower frequences are reproduced well, you're still not getting all the frequences. The range is 35-20000hz. The typical output of a Sound Blaster is 20-20000hz (this can go higher or lower, especially with lossless audio codecs). That's 15hz you're cutting off. It may not seem like much, but consider the fact that a woofer only outputs the lower 60hz, and you'd see that you're losing 25% of your bass sounds.

Now let's take a look at the Creative Gigaworks (which is a great speaker system). It's range is 25-40000hz. Right away you're getting 10hz more bass than the Logitech, and on top of that, you're getting 20,000 extra hz in the highs, which is huge.

Now let's take a look at my preferred listening device, headphones! A Sennheiser HD595 has a range of 12hz-38,500hz. Needless to say, the sound quality is amazing. And that's not even a super high-end Sennheiser pair of headphones.