I've tried both the Meisel COM-90 and the Sabine Zoid Z-1000 clip-ons.
The COM-90 works great with my bass, either clipped to the bridge or the pegbox, either arco or pizz, either open strings or harmonics. I haven't tried it in a really noisy environment yet, or checked the visibility of the display in sunlight, but I'm sufficiently impressed that I'm definitely keeping it.
The Zoid Z-1000 was hopeless with my bass. Couldn't find the E-string with a map and instructions, regardless of where it was clipped, arco or pizz, open string or harmonics. Didn't do too well with the A string either. It got sent back. It does seem to work pretty well for guitars, which is probably their target market.
I think results probably vary depending on the harmonic content of notes on individual basses as played by individual players, bows, etc.