Yep, they're horrible. I never get them during the day, it's always at night. I had an awful cramp a few weeks ago and I was limping for two or three days because my calf muscle felt all tender, sore and strained.
I usually get them as I'm waking. I'm half-asleep, move my leg - and then suddenly a vicious cramp starts. I immediately flex my foot up (bring toes towards front of calf) and try to rub the muscle with my hand. That helps a lot.
I sympathise because they're unbelievably agonising. I think the top tip is to spot them just before they start. I also drink plenty of water and have a reasonable amount of salt, and that seems to help too.