We have marked and unmarked cruisers. We have one "undercover" vehicle. We use a "mixed fleet" approach with both sedans and SUVs. This gives us greater flexibility in poor weather and the SUVs carry more equipment. All unmarked vehicles are fully equipped with emergency lights (low profile) and sirens. Most unmarked vehicles are for administrative staff and serve as our backup to the marked fleet. For example, we had a mutual aid call to Woburn where an officer was shot, all of our marked vehicles went to Woburn to assist. Other officers were then able to use our unmarked fleet to patrol Lexington.
There are some competing interests when it comes to marked vs. unmarked vehicles. There is a growing expectation that police vehicles should be low profile so we do not "embarrass" or cause alarm at certain types of calls. This is completely opposite to the importance of being highly visible for traffic enforcement and as a deterrent to crime.