Students can't seem to agree on this one—some people think that DPS invades students' lives and wrecks their fun, while others believe that DPS only cares about parking tickets and not about alcohol and drug use. Iowa State University does enforce the legal drinking age laws, so if you're underage and looking for a good time, this may not be the place for you. Fines are hefty, and drunk driving will easily result in your license being revoked. If you aren't sober, don't drive. If you're underage, don't draw attention to yourself. Campus police are cracking down on underage drinking, and the fines are expensive.
Most students at Iowa State have few problems with campus police. You can get away with a lot, as long as you are smart about it and don't cause trouble. As long as you don't seek out trouble at ISU, you should be fine. Generally, only really loud parties attract DPS's attention, and as long as you conduct yourself in a semi-sober manner around the campus police, you should be fine.