I have an electronic Korg metronome that has a lot of handy features. In addition to the ticking sound, it has a light. You can adjust the volume so if you want you can turn the sound off all together and just use the light. It also has a distinct "tick" and colour for the first beat of the measure, so you get something like TICK, tock, tock, tock, TICK, tock, tock, tock (or RED, green, green, green, green, RED, green, green, green green).

You can set it to any number from 1 - 9, and it will reproduce that many beats per measure. It also has a "tap in" function where you can tap in the speed you are looking for and it tells you how many beats per minute that is, and then reproduces that speed for you.

I use it all the time!

And to answer a little bit of the initial question, a metronome can be REALLY helpful when learning new material. I often will work on a few measures at a time, if I'm working on a fingerstyle piece. I will work without the metronome until I've got the sequence of notes I want happening, and then I will turn on the metronome at a very slow speed (60 bpm or less, usually), and practice those few measures over and over in a loop with the metronome, until I can play it smoothly. Then I move the speed up a few bpm at a time (maybe I'll try 62 or 63), and do that until it feels smooth and comfortable. Then I'll move up a little more. The trick is to start slow enough so that you can play the passage in rhythm, and then slowly increase the speed-- rather than starting at a speed that you can't really master.