I started out using metronomes for running scales, arpeggios, fingering exercises, reference tempo for practicing difficult passages at slower tempos, working out exercises. Once I'd built up my bass vocabulary and began thinking in terms of grooves, progressions, runs, fills, etc., I discovered drum machines (with and without quantization) and live drum loops were much more inspiring. With the loops, you've got tons of options. Some with a snare behind the beat, some on top, some ahead. After that, band backing tracks for learning improvisation.
If you have an audio interface and DAW software (Garage Band is fine), you can dump your accompaniments into one track and then record yourself as you play along in a separate one. My playing improved massively, since I could critically lesson to my performance repeatedly. Also handy for learning what frequencies to boost or cut to make the bass sit nicely, as well as learning how to use a compressor. For instance, with multi-track drum loops, I used the kick track to trigger a compressor via a side-chain to tighten up the low end in my mixes. When the kick hit, the bass levels would duck a bit.
Plus it's cool to record a few tracks of solo bass to build a montage and play along with yourself. Put a whammy pedal and a distortion box in your signal chain for some cool Doug Wimbish-style pitch shifted shredding.
Or, if you're Jeff Berlin, just take a sledgehammer to your metronome, as he doesn't consider them useful in learning to play. He and Steve Bailey had a friendly, but contentious debate about it. Think it's on YouTube somewhere.