I bought my Stihl FS 55-R last summer. I had about an acre of weeds that were 6 feet high. (dead and brown) I didn't want to run my mower through there because I couldn't see what I might run over. The orange line that came with it was useless. I bought the 3 blade head and a bag of a dozen blades and that got me through that project. They cut well, but I broke a few. The trick is to cut with the edge of the circle and not get too impatient. (Same with the string...)
During normal operations around the place, I found that I was spending more time re-stringing the thing than cutting with it. It was very frustrating. I tried a string that had 3 edges thinking it would cut better, but it broke pretty often as well.
Then I found a string at Lowes, made by Husqvarna, that has a titanium string down the middle. It's silver and costs about 12.00 for a pretty big roll. (Not sure how long) I have found that now, I can run through about 2 tanks of gas before I run out of string and it almost never breaks.