My 6m antenna is a Moxon. I built it because it seemed to be a good
tradeoff over a 2 element yagi, I hear the gain is within a dB or so of
a typical 2 el yagi but the front-to-back is very much more (for a
relatively broad-banded design); the element tips are bent in to
enhance the coupling between the elements and ensure a proper phase
relationship to get good front-to-back.

Mine is very small and light, modeled F/R around 50-50.5 is 25dB or
so, real F/B is s7/inaudible on my FT-857D, for what it's worth. (Not
much, I know. I've moved out of the apartment and into a house with a
backyard so maybe I can make some non S-meter based measurements). I
think it's worth considering vs. a 2 element yagi for HF. You'll
notice the enhanced directivity more than you notice ~1dB of gain.

On 6m, I was just looking for a very small, light array for an
apartment balcony antenna, and I liked how the Moxon stacked up. I
didn't really think the F/B was really going to matter until a security
light went on the fritz during a nice opening... the band was open out
west to Idaho, California, etc. The light was to my east. Pointing at
it; s9 noise, pointing away with the light in one of the back nulls,
nice band opening, no noise! Since it has a broad forward beam,
beaming off 10 degrees to get one of the -35dB nulls in the back on the
noise source didn't affect the desired signals much.

OK, so, dumb "I worked this cool stuff with it" anecdote aside, mine
does what it claims to, and I think it's worth considering, especially
for the lowest band on a compact multiband array. It makes it more
compact and increases the F/B ratio over a 2 element yagi. The extra
directivity makes it work very well as a beam psychologically and
emotionally, and there is a even a modest actual improvement to signal
to noise, moreso if you have a real noise problem in some direction and
like beaming the other! At the apartment, when that light wasn't
acting up, beaming to Europe put my building and the other closest
buildings off the back... there was a substantial reduction in the
noise floor when I was pointed that way...

I'm considering one for 20m now. I have permission to put up some
antennas at the new rental house but I don't have a ton of space. You
can make one with fiberglass fishing poles and wire that's quite light
and can be rotated with a TV rotator, though I wouldn't trust it to
survive a major gale. www.moxonantennaproject.com has a bunch of
pictures and descriptions. Mine is at www.n3ox.net/projects/sixmoxon.

73,
Dan