It depends on the situation and, though I feel kind of bad saying it, my evaluation of the person looking at me. If I'm out on the street, and someone gives me a once over, and seems to enjoy what they see, but just carry on, I consider it a compliment, and appreciate it.
However, if the situation is more static, like on the train, it depends on who's doing the looking. Whether I appreciate the look depends mainly on my assumptions on how a potential direct social interaction would go. If the person seems to have decent social skills, and would carry a normal conversation, I appreciate it, and possibly even open myself up to a conversation (but start one, NOPE). If I assume they lack social skills, I feel super awkward about them looking at me. I mainly feel this way because I have accidentally made eye contact with one such person, who opened up a conversation I did not want to be part of, but as it was on a crowded train, I couldn't just leave (in the instance that comes to mind, I didn't want to get off as I had a lot of stuff with me, and it would have been more difficult than dealing with the conversation).
So, really, I don't mind, and sometimes enjoy, the looking. What I don't like is the occasional uncomfortable social interaction that could follow.