If you have pets or are otherwise cleaning up lots of spills, I recommend getting a handheld one for spot-cleaning.

If you're doing whole rooms, I recommend renting one, as they work better than most of the consumer ones you can buy. I've rented from Home Depot before, many big hardware stores have them.

If you're on a budget, ask around to friends and family to see if anyone owns one. Getting them a six pack of their favorite beer is cheaper than renting. The performance isn't as good as commercial ones, but it's still better than nothing.