seo expierience with w3c (in)valid sites

i have downloaded some crappy niche site packs , they are not completly valid suche i can change something here and there, but not everything.
Had anyone ecxpieriences with invalid sites and google indexing/ranking etc..?

Yes and no. If your niche site has some sort of error which never lets the browser finish loading then Google would mark the website as slow and penalize it in the SERPs