I just bought a new Stihl FS90 with the 4-Mix engine and the "bicycle" handlebars this last Tuesday. I also got one of the chisel tooth saw blades for it as well
I was really skeptical about the 4 stroke engine, but my dealer said everyone that has bought one from him really likes them, and so far he's had no returns of any with problems. So, I plunked down my $$$'s. Got home and put my polycut head on it, and went to work. All I can say is "Wow!" I absolutely love it. Much quieter/more pleasant to listen to than a 2 stroke, and it has PLENTY of RPM and power... I rarely, if ever had to use over 1/2 throttle, even cutting the heavy fescue ditch banks along my road. I ran 2 tanks of fuel through it the day I got it, and then ran another tank through it Wednesday.
I've had Stihl's ever since I was a kid (dad bought an FS80 back in the late '70's) and I've got an FS44 and an FS80 myself, and now the FS90. I don't think there's any better than Stihl as far as trimmers. So, you ask, why did I get the FS90 with those other Stihl's? The old ones just have the loop handles, and I really wanted a bike handle unit like Dad's old FS80... much less fatiguing...
I've only got 3 tanks of fuel through my FS90, but so far it's an enthusiastic two thumbs up.