I was satisfied with the Digitech Bass Synth Wah for octave-down sounds. Mine stopped working after a few months of using it at practice once a week, for one song, and the store I bought it from had thrown out all the paperwork (which many stores seem to do with anything they open up for display) so I was out of luck. Could have just been a rare lemon though, haven't found many complaints online.
I'ved used the Akai Unibass octave-up pedal, owned 2 and sold both for more than twice what I paid. They track well, and produce an octave up sound that, depending on the settings, can be strange but kinda cool or just plain annoying.
Just last week I plugged the headphone out of my Roland Bass RX Micro Cube into my amp, and it performed pretty well. I didn't try to test its limits, just played a couple of songs on a 4 string and it didn't seem to have much lag.