I recently purchased a pair of Grado i60's to replace my broken Sennheiser HD420's. I like the sound, and supposedly the only difference is only a certain percentage of 60's get graded to become 80's (rumor has it). Not as light and comfortable as my Sennheisers, but that is just because I was used to having almost nothing on my head (I would always forget I had the damn things on they were so comfortable). I like the Grado's. I tried a pair of Sennheiser 515's at the same shop as the Grado's, and I felt the Grado's were more comfortable vs the 515's. Sounded very similar, I really couldn't tell the difference between the two, so I chose the Grado's, at roughly a $20 savings.
Again, it is all subjective. It depends on taste and comfort. Definitely worth seeking out someplace that would let you audition them before buying, if at all possible.
Neil