Not bright, not a good treble in two words:)
Sennheiser IE60 Review - Page 3
Head-Fi's Best Sellers
i just got these couple of weeks ago, and the bass just keep on getting better the more i use them, i would say treble is bit rough but not by much. also the isolation is EXCELLENT i can listen to a moderately high volume and still its leaking almost nothing. Bass is also punchy and detailed at the same time, one bad thing wich i really dont like is the seal, they are pretty hard to get good seal on and also they keep popping out from your ear if you run or ride a bike for example, but for sitting or walking they do mediocre.
The ones I use are the Ultimate Ears single flange tips meant for the TF10. They fit the IE60 and my ears rather well.
http://www.logitech.com/en-us/ue/ue-earphones/accessories (grey tips near the bottom)