I've owned some PX100s for 3 years or so, and used them with a portable CD player and more recently a 4G iPod Nano. I love their natural, open sound and think the leaking in of external noise a small price to pay. Maybe I'm biased but I've always thought their sound well balanced so when a friend of mine bought some CX300iis (Sennheiser again) recently I was surprised at how bassy they were. Perhaps it's the nature of buds (I'm not a fan) but on a straight comparison using my iPod I felt that there was just too much bass, leaving the mid and treble seeming quiet. I could use EQ settings to reduce the bass I know but I prefer to leave settings flat. For balance, my friend is thrilled with his new purchase however. He loves the "in your skull" sound and I'm sure the extra bass is right up his street.
So that's a vote for the PX100s from me