Had an acoustic trio in the early-90's that wanted to experiment with some of the then-novel string sounds that were appearing on the market, bought a Bozeman-made Flatiron 3MC octave mando - not the best one out there but a well-made, good-sounding/playing instrument that allowed us to explore some Celtic and neo-folk material (including an arrangement we had of Dan Ungar's well-known "Ashokan Farewell" that used all double-strung instruments - mando, octave, and 12-string guitar); a few things I learned:
- My/our needs would have been better served with either a mandocello or a then-new baritone guitar - not enough low-end extension or volume (especially given the small/thin body) to provide a solid foundation, too much overlap with the guitar's range, and can easily get lost when things get cranking at a seisiun (Irish tenor banjo covers the same range with considerably more volume) or other high-energy acoustic setting;
- IME it's a niche instrument as best, and as previously suggested somewhat limited in scope - you need to buy a good one to obtain any kind of real tone, and that at the risk of it becoming merely an expensive toy (remember the sitar craze of the mid/late-60's?
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- Frankly, mine hasn't left its case in close to a decade (no joke), and since it's extremely hard to sell/trade/consign I've considered having the bridge and nut recut/replaced, and converting it for Greek-style bouzouki in the interest of continued utility: far more practical for my needs, the body size is more appropriate tone/volume-wise for the higher tuning (I'll be using DGBE, a step higher than traditional Greek CFAD), and it makes an interesting visual statement on stage - only drawback is that the neck is a little wide for its intended purpose...
You live and learn and, knowing what I know now, if I had it to do over again I'd never buy an octave mando - YMMV...