I like the HD555. A great introduction to hifi and also excellent as portable headphones. They might look dorky, but with their relatively low impedance, they work great with the iphone 3g. As long as you only use them for walking around in quiet places or at the park,they are the best portable headphone IMHO. Being open also means that if you use them outside, you can hear noises around you, which is actually good. With closed headphone you feel shut off from the rest of the world. I understand that in noisy places open headphones would be unsuitable, but if you want to listen to good music when walking along a beach or sitting in a park they are excellent.
I have upgraded to HD650 (not be used portably LOL) and I can hear a big difference. The HD650 have so much more Bass, transients and detail that it is hard to compare in the end. Higher End Audio is always going to cos that bit more. I did hear the Famous HD650 veil but when I upgraded to a Class A amp that disappeared and the HD650 actually sound bright!!!! If you like the Sennheiser sound it is a good stepping stone. The next step up from the HD555 is a headphone amp. Even a $150-200 Headphone DAC/AMP will make a huge difference such as the Fubar III. I have owned Sony & Yamaha headphones before so am not a 1-eyed Senny fan, but I do like their sonic signature.