Solid Hardwood, Engineered and Laminate Flooring - How good is the attached underlayment on Pergo floors?

Is the attached underlayment on Pergo floors any good? From what I'm reading it's not advised to put down another layer of underlayment as it may cause problems with the laminate but I want to make sure the stuff on the bottom of the Pergo flooring isn't the equivalent of the cheaper underlayment that's sold seperately. Is it at least equivalent to a mid-level underlayment?

Welcome to the forums!! The attached underlayment poses no problems as it provides a (for lack of a better term) solid cushion that won't shift. You are correct that no additional underlayment is used. It will make it too spongy and could harm the flooring causing it to split at the joints. Pergo warrants their products as well as any, and their underlayment is far superior to the roll out foam.

Ditto. Pergo's attached underlay is more of a rubber as opposed to the cheap open-cell foam...will provide a much better sound and feel than the cheap stuff.

Thanks for the responses guys. Quick question about floor prep. The old flooring was carpet over a concrete slab. I'm pulling up the carpet tack strips and there are of course holes from the nails in the tack strips. What sort of filler should I use on those? I don't mind mixing something myself (as opposed to ready to use premixed stuff) but I'd like to avoid buying a 40lb bag of cement to fill a couple of holes if possible.