Overall, cougar attacks are becoming more common given increasing human encroachment into mountain lion territory; all things considered, though, attacks are still exceedingly raremaybe between a dozen to 20 per year in the United States. Your odds of being killed by a dog are ten times as great. (Need anecdotal evidence? I have a large scar on my right thigh where I was bitten by a rottweiler while bicycling a year ago.) But, mountain lions do have one trait that bears do not: They'll actually stalk and hunt a hiker or mountain biker, attacking at an opportune time.
Pepper spray? Sure, cats have very sensitive noses, so that will work. The trick is hitting a very agile, fast-moving animal if it decides to attack. And, cougars are stealth hunters, so you might not even know it's there until it's too late. For peace of mind, though, Counter Assault is available from REI at $38 a canister (www.rei.com).
Overall, I think it's prudent to be wary when in cougar country, although I suppose it's also prudent to worry about rattlesnake bites, frostbite, poison ivy, bee stings, Lyme disease, and a bunch of other hazards that still remain more likely. In other words, pull this string and pretty soon you can't keep track of all the things you need to worry about.