L&O is one of the most reliable shows I know in terms of just being able to tune in and be entertained. A lot of it can be pretty formulaic, but it's well presented and generally deals with interesting cases. I also like the dual approach of following the investigation first and then moving into the trial and everything involved with that.
I will say, however, that I usually prefer the police part, somehow. They've got pretty good teams going there, and some of their banter can be pretty funny. It's also interesting to see how much closer they are to how things really are whereas the attorneys have to deal with the reality of the judicial system.
Out of all the L&Os out there, I tend to like CI the most, although I enjoy all of them. I just think D'Onfrio is great in his part and really stands out because he's kind of unusual in the way he approaches cases and people. They do stretch what he gets away with at times, however, which can be a little annoying.