It cost me $1600 to have the head gasket replaced on my Accord last summer. That included having the head machined because it had warped.
That is a lot of money to invest in a car. If the rest of the car is in solid condition, and the price is dirt cheap it may be worth if. If you are capable of doing the labor yourself, that makes it more likely to be a good investment, but you have to look at the total cost of the job, plus the car, and compare that to what it would cost your to buy a similar car, in like condition, but without the bad head gasket.
If you could get a similar car without the problems for about the same as it would cost to buy this car and fix it, then I would say it is not worth it. If a similar car would cost hundreds more, then perhaps it is worth the cost of fixing this one.