If the price is right it always make sense to buy. You just want to get it less the cost of all the repairs and less a little hassle factor.
So if a house is worth $200,000 fixed up and needs $15,000 in work, don't pay any more than $185,000. Because after all you'd still be paying full price. A house with that kind of work needed (the 15k number) you'd want to get for $150,000 - $180,000. Obviously the lower the better a deal for you, but you should still try to get a discount for the fact it needs work. You want to build in some equity for all your hard work.
Now that is assuming you're buying with a bank loan. You have an upper hand on the fixer because you can get a loan. But if the seller is offering some type of financing, whether owner financing or a rent to own, you couldn't expect to get such a discount. If they were financing you, you'd probably need to pay the full price because they are offering you an opportunity to own.
Just don't over pay for a house that needs work. And make sure you have the reserves in the bank to afford those repairs. Buying a house is expensive, last thing you wan to do is spend every bloody cent you have and not be able to fix the house up for months or years later.