Vicki: Be aware that Insuring a home with Polybutylene Plumbing may be diffcult . . I had a client once that after we found out that the insurance will put a cap to limit coverage. . I adviced him to walk away.
You Realtor should insist to get it fixed or walk away. .after all, the chances of them selling the house to someone else will end up with the same problems.
Get it fixed by the seller or walk away. .
Get it fix and split the expense with the seller if you really want the house.
GET YOUR FEET WET!
Take a chance in buying it AS IS and . . it will be a matter of time for the pipes to burst open, I heard a plumber tell me once. . that PB pipes will have a 90% failure after 20 years. . . not very good odds

It depends on a lot of things. Are you getting a good price one the home? Do you love it no matter what? Would you be willing to replace the plumbing when needed? Historically the problem with this type of plumbing is that it will start cracking and cause leaks throughout. This doesn't always happen at once and may be more economical to replace as needed instead of all at once. It really matters what kind of condition it is in. You migh tbe able to get the seller to credit you for the replacement but they could also say this is an upgrade not something they need to take care of. Did you find this out during your home inspection?
I would talk to your agent and voice your concerns. But educate yourself first on the condition and see what the real costs are going to be to get it replaced.
People often get scared too by galvanized piping but it may last a lot longer and you an just replace it as it goes or all at once. It just depends on your comfort level knowing this. Obviously this is an older home, so look at the overall picture. Does it also have a leaking roof, mold, a wet basement, termites, anything that might add to the repairs down the road you need to make? Most houses, even new ones, will have some sort of issue that needs to be addressed. But educate yourself first by talking to a plumber and several at that! to find out the real cost of correction. You might find it is something that you are willing to deal with or not, but at least you won't be walking away from a home you love, to find out later it would have only cost a few thousand dollars and now you can't find something else you love just as much. But ask the seller to help pay for repairs, or reduce the price.
Your agent is there for you and you should talk to them about your options and the situations of the sellers. If it is as-is, then you probably won't get any credit. But negotiate and then re evaluate. Just make sure that you are going to be comfortable with whatever the outcome is. JUST BE EDUCATED then you can't make a wrong decision.

Vicki,
We had a client buy a townhouse with polybutylene knowing that they would have to deal with it one day. After 4 years they replaced the plumbing for about $4000. There is a certain kind of polybutylene that is not a problem, we had it in our first house and never had a problem, neither have the present homeowners, that's a total of 19 years.) you need to have a certified home inspector tell you what you are dealing with.

Hopefully you put in a home inspection contingency when you made the offer. If not, there may be nothing you can do.

If the seller is open to it though, you may be able to negotiate something; the market is so competative right now that a seller might be willing to condider something rather than loose a contract.

Again, it depends on what the contingencies are. Hopefully your agent is a good negotiator. Good luck.

Did you know about it at the time of offer? Did you have a home inspection contingency as part of your offer? Did you have a home inspection? If so what did it say? These are all things you need to explore with your real estate agent or a qualified real estate attorney.