I use a Line 6 Variax 5 string which has good 8 string and 12 string models which I use a lot. When it was nearly destroyed in a car accident last year, I tried a LOT of octave up pedals and was never satisfied with the result. I tried 3 Boss boxes, a Behringer knockoff of the new Boss harmonizer, Zoom box, several rack effects...but they just didn't sound right. The octave up was a bit digital sounding, often sounded out of tune (even though a tuner showed it spot on). Usually it sounded like a Farfisa organ. I even tried a 300Hz highpass filter before the octave box, which works well for effects through a separate guitar amp (in addition to my regular bass amp.) This worked better, but not perfect.
I finally fixed my Variax and I'm a happy guy. I did not try the Pog or Micro POG, but I suspect it would be better, but the cost just to try one is prohibitive to me. I'll stick with my Variax...until I modify my Squier Pro Tone 5 (3/4" string spacing) into a 10 string.