My wife and I booked the Glacier Walk (on a really short notice - thanks, Oddur!) with Melrakki, and it was beyond perfect. Katerina was our guide, and she was really professional and knowledgeable (we felt safe the whole time, and she knows the exactly best spots on the glacier), but at the same time very funny and light hearted. Oh, and the glacier is out of this world, you won't believe it.
This is a relatively new company, but don't be afraid to book with them, you will be in great hands. Since they are a little smaller, everything will be more private and personal (also, supporting new business is ALWAYS a good thing to do).