Most mechanical relays and switches are rated for 1 million cycles or less. That sounds like a lot at first, but if you have a sump pump that runs every 30 seconds, that's less than a year. Lowes used to sell a sump pump with a lifetime warranty. Every year, the manufacturer would mail me a new switch, and about every three years, a new pump, until they finally insisted on refunding my money.
Warranties on all the new sump pumps I looked at are awful - most are one to five years, and they expect you to mail the pump in each time. Some don't warranty the switch at all. Not keen on replacing the switch every year, and not wanting the basement to flood, I decided to build a solid-state arduino-controlled system. Water level will be detected using eTape, and the pump will be controlled with a solid-state relay. I've begun putting together a prototype:
