I've heard bad things about HP/Compaq/Gateway quality as being cheap and flimsy. HP owns Compaq and Acer owns Gateway and they both have had some pretty bad reviews.
I think ASUS and Toshiba have the best quality. IBMs are well-known for their reliability, but I'm not sure how Lenovo's acquisition has affected it. Dell varies from system to system, but my personal experience has been good with them (CompleteCare ftw!).