Yes I use mine 3 or 4 times a week. I'm working from home so I just prepare it in the morning, then it's ready for when I finish work. I've just looked it up, and it averages about 1p per hour ("average" energy tariff UK, and I suppose "average" sized slow cooker). So 3p an hour worst case scenario I would think.
One general tip: It's a lot quicker if you put hot stuff into it first, especially if you're going out and intend to leave it on low for the whole day. If you're staying in, you can put in on high for the first hour or two and then turn it back.
For example, I'd brown all my meat on the hob first, then transfer the meat into the slow cooker. Then I'd use the same pan (with the meat juices and charred stuff) and put my veg in there on a high heat. Then when the veg is hot, transfer that into slow cooker. Finally if I want a stock or a sauce, I do that in the same pan, get it near boiling and pour it in the slow cooker. You can then leave it on low.
My partner put a load of cold (chilled) meat in hers with raw veg and left it on low from the outset. After 4 hours the carrots were still crunchy and the overall flavour was "underdone". So avoid that. Oh and I switch mine on whilst I'm preparing the food, only 10 minutes early, so it heats up. but you musn't leave it get hot for too long with nothing in it.
Ideas:
My favourite is a stew: meat of choice, swede, onion, carrot, parsnip, potatoes if you like, garlic, seasoning, brown first and then put in slow cooker as above. Make some stock, get it hot, then pour in. Half an hour from the end I make some dumplings and put them in, put the lid back on. You can also drain your gravy into a pan and thicken it on the hob if you want it thicker.
Curries - brown meat with spices in a pan, then transfer to slow cooker. soften the onion and garlic (and chili if you like) in pan and transfer. Heat up coconut milk in the same pan, then tip that into S/C. Add sugar and salt to taste.
Spag Bol/Chili : same principal, brown meat first, then veg, then pour over sauce.
A good tip for anything if it's too runny, carefully take the lid off and slowly move it away to the sink, tip it up and loods of condensation will run off. Keep doing that every 30 mins , and you'll have removed half a cup of water in just a few goes.