I casually watched the first episode when it was first broadcast. I was hooked from the start and did not miss a show that season. Criminal Minds is set in Quantico VA at the FBI's Behaviorial Analysis Unit. It is a team of profilers who analyze evidence to discover who the "unsub" is (I think it is unknown subject). They travel around the country helping local police with difficult cases.
The scripts are suspenseful, the characters are both skilled and flawed. The finale was intense and a real cliffhanger. The whole season was very well done. It is a must watch for me on Wednesday nights. I could not wait for season two.