what is the benefit from buying dao tokens with eth???
If you want to buy during the pre-sale, ETH is the only way to get some. That'll change once the pre-sale closes and The DAO is listed.
ok from what i understand it's an ether investment that's right ?
It is during the pre-sale. In a sense, it always is because the successful Contractors will receive ethers to fund their approved proposals and will be obliged to pay out ethers once they make money.