A Nucleic Acids Research paper (Vol. 20, No. 14 pg 3551-3554, 1992) documents that the 3' to 5' exo activity of these proof reading enzymes will degrade the amplification oligos. Their solution is to use phosphorothioate primers to protect the primer from exo degradation. If you don't really need the fidelity (a marketing ploy?) stick with what works.