I did it a couple of years ago and found it really interesting.
There were two trials, one lasted a couple of days and one went for 2 weeks. I got chosen for the 2 week one. I met some really nice people and I quite enjoyed it even though the trial was a bit mundane (benefit fraud).
Didn't find there was too much waiting around. They were pretty quick with processing people.
All in all I found it good and I just really wanted to do it to be part of a jury (and to get a couple of weeks off work because my boss was driving me up the wall!

) , but probably wouldn't do it again.