I've been called, but pretty much disqualified because of my occupation.
Experiences can vary. You may be fortunate enough to be empaneled on a criminal trial that has a real victim, so you can feel like you're doing something good and meaningful for the victim and society in general. (That's not to say that on a quote-unquote victimless case you won't feel good... One way or the other you're there to "Do Justice.")
Basically, you'll just be called upon to assess the credibility of each witness- police officers, victims, eyewitnesses, expert witness (at least if it's a criminal trial)... Try not to let any biases, in favor of OR against any witnesses, affect what you see and hear in court.
Having said that, you can do some good on a civil trial too -- it just depends on the case. I'd go with an open mind and be honest about whatever you're asked... If it's a criminal case, you'll probably be asked about any past experiences with the criminal justice system, police officers... were you or any family members the victims of crimes, did you call the police, how did they handle it, was there a trial, etc... Again- whatever your answers are to those questions, the ultimate question is: Can you put aside any positive or negative experiences in the past and just assess the evidence (including not only the testimony of the witnesses but also their demeanor in court and their forthrightness, etc.) that's put before you in this trial...
Oh, and ElectricSheep: In New York and California, you can not be called for jury duty IN ANY CONGRUENT JURISDICTION within a four year period. So in these two states (and Jersey is probably the same as well), if you get called for state/city or federal, you're exempt for four years... And this covers both trial parts as well as grand jury duty. (That'd suck if you can truly be called as often as they need!!!)