So, I figured I would go with what I knew (Sennheiser), and the CX-300's are pretty decent so far. I only gave them a 10-hour period for burn in, and then used them overnight (My room-mates are pretty loud) at low volumes, and the sound is still changing from then. But, so far, there's nothing I don't like. I think my PMX-100's are better for running, due to the microphonics thing, and that they stay on your ears easier, but I think the CX-300's are clearer and more detailed. This could just be that some of the details I'm now hearing fit into the PMX-100's high-end valleys.
I also have the creative Aurvana Zen iem's (absolute junk for any more than $20), and the CX-300's don't have the same vacuum seal, but they still seem to both stay in better and give much better bass. The highs on the CX-300 aren't as rolled off as the Zen's, either. The Zen's sound clearer, but that is probably just due to their heavy mid-section.
Oh, and I Love the bass on the CX-300 (not the tempation to penetration kind, though; I love the sig), it's nice and tight, but with good presence. I think I might lose some of it's control as it continues to burn in (that's what happened with my Creative Fatal1ty headset, it sounded perfect out of the box, but turned crappy in less than 20 hours of use.