FI is obviously not a very often used airline, but there are a few posts laying around. Here are some links to at least give you an idea of what to expect... basically, nothing good or bad, loads are generally reasonable:

http://nonrev.net/europe/12656-icelandair.html
http://nonrev.net/europe/14277-fligh...s-iceland.html
http://nonrev.net/europe/14205-icela...periences.html

I just hate the idea of how they use a 757 on a long route like MSP-KEF, but I guess that beats SY's new MSP-YQX-STN route on a 737.