With octave pedals YMMV is the name of the game. The bass you use, the strings, and more than anything your technique have MASSIVE influence on how well one unit tracks over another. I tried out both the Aguilar Octomizer and the MXR BOD over the last several weeks at Premier Bass Guitar, and found that with some basses the Aguilar tracked lower than the MXR, switch basses and the MXR tracked lower...
This is the most important post of all the replies so far (no offense intended otherwise, everyone has their thing), but truly, each of the octave pedals will act differently based upon all of the variables mentioned above.
I personally had an octave pedal before I even owned a pedal tuner, and if there was ever just one effect I would never be without it would be this one.
I have tried darn near every bass octaver that has come to market in the last couple decades, and the OC-2 and now the OC-3 are the ones I always come back to. Some people think they're crap, but they work the best for me with regard to my most commonly used basses+strings, but
most importantly the way my fingers hit those strings.
You've just gotta try some different ones and find what best works for you. One more thing, if you plan to use the octave pedal in combination w/ any other effects, also be prepared to test the pedal(s) at different points in your chain before you settle on one, as some pedals don't play nice with other ones sometimes.

With regard to the money aspect, spend a few moments online to find the retailers that offer free shipping if you want to try certain pedals that aren't available to you locally (most big music houses offer free freight for purchases over $100, and any decent octave pedal will almost certainly cost you that much, if you buy new anyway); at least that way you may only have to pay return shipping.