Jacob F, I too used Fireline for nearly a year. It has a small diameter. It is really strong. It does cast well. But experience has convinced me to go back to the U.G.. First off Fireline has zero stetch. I used to think that was a benifit, now I don't. I recently read an article in STS that expressed exactly what I found out the hard way this fall. I lost several really nice hooks that I probablly would have landed with the help of some stretchy mono. Second the Fireline quickly begins to fray on the first 30 yards that are always out of the reel. When it begins to get frayed I found that knot strenght is dramatically reduced. Keep these things in mind when using Fireline.
Ass far as using a blood knot to join new line to my reel, I will look into buying the U.G. in bulk, because I would hate to loose The Big One. But like a poster above said fly guys use them all the time. I have never had a fish take off 75yrds of line, but I would like to have one do so. That way I will find out for real if a blood knot is the way to go.