I am a drummer by trade, but also a bassist, and was happy with using phone and iPad apps to use as metronomes. I also felt that they all lacked in particular areas and there wasn't exactly one good one that did everything. Also, running these apps drains battery and using your phone or even tablet with headphones can be cumbersome, especially when its purpose is only to be used as a metronome.
The Boss DB-30 solved all my problems straight away - it's extremely light, has a headphone socket, has a pleasant sounding click, has multiple timings and a large tempo range. One of the great things about this (as a drummer) is the pitch function that allows me to have a base pitch when tuning drum heads.
Other than that, the built in speaker is great when I'm practicing bass, although you can't hear it at full volume against a snare drum. It perhaps seems expensive for a digital metronome, but it's the only one you'll ever need.