There are excuses for not doing it, as others have said. I once got let off because I had a booked holiday. I said I'd do it when I got back, but they didn't bother. I did get chosen again a few years later, and did it. It was very interesting. You should do it, as Eddie says: if you think the jury system is a good one, you should be prepared to do your it. (And even if you don't think it's a good one, and all accused should just be taken out and shot, it's still the system we've got, so you should still play your part.)
Incidentally, you don't 'pass judgement', you just decide on the facts: listen to the case for and against and decide whether you think he did it.