Most don't work if coolant and oil are mixing. It really depends on the nature and severity of the leak.
I'm actually using it in a vehicle I drive regularly and have been for about a year and 6,000 miles. I dug my Explorer out of a tow yard inop, no key, and decided the body was too good to just part out. Once I got it running, it was blowing exhaust out of the coolant reservoir, so I decided to try something in a bottle since it was a $250 vehicle that I still wasn't sure about fixing up/driving. Decided on K-Seal, the big bottle, and so far it has worked like a champ. I may eventually do a proper head gasket replacement (vehicle has NO rust), but not before the K-Seal quits working. Heat works great, engine runs great, and I don't even get little bubbles in the reservoir. Will it last forever? Probably not, but in my case it worked exactly as described and far exceeded my expectations. I completely expected to be throwing away $35, but it has bought me at least 6000 trouble free miles.