I have a 1-year old 4-MIX Stihl BR 600 backpack blower, and I just did a valve adjustment on it myself. It's very simple to do, and only requires the valve adjustment tool ($1), and the valve cover gasket ($3-4). The valves were just slightly out of adjustment after a year's time, and from what I have heard the 4-MIX trimmer engines don't get out of adjustment as easily as the blower engines. It only took about 15 minutes for me to do the whole procedure of taking everything apart, adjusting, and putting everything back together. I have heard it is supposed to be done once a year just to be safe. I am also ready to switch over to Amsoil Saber, so that I won't get any carbon build up. It has been run on Stihl Ultra until now, and hardly any carbon has accumulated, but the Amsoil is quite a bit better. In the end, I think that the 4-MIX engines are highly reliable when maintained, and the maintenance is very minimal. If you read on this site, most people that have both the FS 80 and FS 90 or 110 say that the 4-MIX is way better than the 2-stroke FS 80. Much easier starting, a lot more torque, and isn't so loud. I have used both an FS 80 and an FS 110, and the 110 felt lighter than the 80 because it had good balance. The 80 is heavier in the line head end, making it highly unbalanced. I would recommend the FS 100RX, and put a solid shaft in it too. It's a VERY light trimmer, with excellent balance.