You should use a UPS if you have many thunderstorms in the area. That will save hardware, software, your time, and money, and help prevent you from throwing out that old monitor, CPU, or modem when it gets trashed by lightening. For details consult the UPS-HOWTO.
UPSs do save hardware, save work, etc. In areas prone to lightening, they could save hours of work each week, potentially translating into power savings. They do save hardware in areas prone to power outages. There is some concern over their additional usage of AC power. If anyone has any studies or research on this issue, could they please forward it to us? That would be an interesting question to post to APC, BEST, etc.