Check out the Eleca Black Hurricane, it's a 30$ clone of the Multiplexer. Have had mine for awhile now and love it, haven't had any issues. The original EHX ones show up fairly cheap used sometimes but the Eleca is great for trying it out cheap. Also EHX makes deluxe multiplexers but they are old and expensive, but seem really really cool. Hopefully someone clones them too one day or EHX releases a new deluxe multiplexer... The simpler version is still great though.
If the OC-3 is polyphonic I'd assume it is digital, nothing wrong with that but there are a lot of other great newer digital octavers with more controls for the price. I hear good things about the OC-2 and the signal controls seem great, but I really love the dual filter and simplicity of the multiplexer. The blend is really all you need and then you can fine tune the octave over a wide range from thick/deep to more raspy/bright especially with the low filter switched off.
The dry output also seems pretty useful for recording, on guitar you could split your signal with the octave into a bass amp and the dry into a guitar amp. And with bass or synth you could send the octave/dry through different effects/processing. Been meaning to try using it into a mixer with the dry/wet in different channels for individual EQ's, seems great for experimenting. Have it early on in my chain right now though so the dry out hasn't gotten any use.
I mainly use the multiplexer on guitar and am impressed how bass like it can sound when dialed in right, I'd bet people listening to recordings or in an audience couldn't even tell it is actually from guitar. But it is also great for synthy stuff, and on bass can be thunderously deep. Also can be fun on synth/keys too and would be perfect for mono bass synths.