You can never learn too many instruments! Though, as godinpants noted, it becomes quite tiring on the expense budget after a while. Like many others, I started on guitar. I've been entirely self taught, but in retrospect learning piano would have been the most useful to start on in the long run. The key is listening...there are a lot of guitarists that play bass like a guitarist when they switch over. It takes a while to develop actual bass technique, not just knowing where the notes are. Of course, it's a matter of intent too. For some people, the goal is to be the best player at their given instrument. For others (I fit firmly into this one), the goal is to be able to play as much as you can, as well as you can. These days keys and electronics are my main weapon of choice, but have learned (in approximately this order) guitar, bass, mandolin, dulcimer (both lap and hammered), harp, psaltry (though if you can play a harp, you can play a psaltry) and fiddle. I can fool around with a few more (I know the basics of accordion, but can't play it well yet), but I'd say the toughest thing for me to learn has been voice. You can figure out guitar if you fool around with it long enough, but it's easy to learn to sing incorrectly, giving you bad habits that eventually have to be beaten out of you if you want to really sing well.