I was always surprised (well no, not really because most people do this) that you would consider taking her back. If someone cheated on me they'd get the boot no matter what.
I think it says a lot about a person's self esteem and how they think of themselves when it comes to situations like these. Often, you see the guy crying and crying (or girl) ("no noooo she would nevverrr do that to meeeeee..." or "She didn't really mean it and won't do it again...," etc).
If someone cheats on you and you have any self respect, you'd drop them instantly.