It can also void your warranty, and if things go wrong, you can brick your phone, so there is a risk. As was already said, it also makes it easier for malware to gain unwanted access to your device, but if you use SuperUser, you have to approve root access for each app. Be very selective in what you approve, and I also suggest requiring approval upon app updates, so that you can make sure permissions haven't changed.
Taking into account the risks and rewards, all of my phones are rooted. I have bricked one before though, so those risks I mentioned aren't just theoretical.