I have owned a few different autotuners. They are a great convenience,
but not without some cost in performance. The reviews I have seen on
the efficiency of autotuners can be depressing. In fact, I have seen
insertion loss figures as high as 65% on some bands and some crazy
impedances.
I think this is due to the use of toroids and fixed capacitors in
autotuners. A good efficient tuner usually has high Q air inductors
and well made variables. Maybe that explains it, maybe not. However,
they sure do work nice! I used an old SGC at the feedpoint of a 30
foot homebrew vertical. It was a kicker on all bands from 160 through
6 meters! Now, that of course is my own anecdotal experience, and the
antenna may have been inefficient as hell, but it got me on the air and
I got out real well. I have also used autouners on random wires and
such, and again they work well. But expect to lose a fair amount of
power.