I have only had them for about three months now and I have probably spend about 300 hours with them at least!
I absolutely love the bass in these headphones, I have no complains there and they do not leak much sound so they are absolutely great as "on the go" headphones. They only problem is that they are not  
really good when it comes to other kinds of music but I would really recommend them to people who likes bass heavy music. They are rather similar to akg k518 except from the softer earpillows and better bass.
Edit
I would just like to edit that the earphones frame is a lot easier to brake than the k518(my k619s frame broke recently) but it is not a problem if you are carefull and do not borrow them to your classmates!
I would buy a new pair if these broke because this bass is so damn good and it just really suits my taste.

Before you read this you should know that I have only tried them for a couple of minutes and this is my experience of them under those minutes and this is all from my own preferences!
 
The headphones is overall good and the sound is balanced BUT! They are a little to balanced to fit my preference. As I said the sound is really balanced but the lack that little extra to make me get that feeling of overwhelming pleasure. Most of the reviews that I read through before I tested them said that the headphones was a little more bass heavy which of course made me happy since I like that little overwhelming bass since I listen to rather bass heavy music. I am sure that they are really good for those who listen to classical music but I would not recommend these headphones for those who like bass heavy music. The reason to why I gave it three in rating is of a different matter. When I talked to the employees  from Sennheiser they told me that momentum was the newest in sennheisers "on the go headphones" witch I think is wrong. Perhaps it is my head that is to small but I think that they leaked way to much to be a good pair of headphones when you normally travel with the subway.