is there a substantial difference between dvd players for the pc. I have an off brand dvd player and i'm not impressed with the playback quality at all.
Tom, I think maybe you're misunderstanding your question (then again maybe not). A DVD-ROM is a piece of computer hardware into which your DVD gets placed. A DVD player is software that decodes the DVD signal (although in the olden days, DVD players (the decoding) were done on a hardware PCI card (Hollywood Plus, Mpact, etc). Due to the increases in processing power, however, hardware decoder cards have become almost extinct.
Judging from your question, it seems like your asking about the quality levels of DVD-ROMs. DVD-ROMs do matter, but not in terms of picture quality. Issues like stability, reliability and noise are relevent when choosing a DVD-ROM and do vary between models/manufacturers.
DVD players (software decoding) do definately impact picture (and audio) quality. The two most popular DVD players are WinDVD and PowerDVD. In my opinion (and in most knowledgable people's opinion), neither one of these can be considered the best. Getting the 'best' picture quality possible, ie the best DVD player, can be a very tricky setup. Overall, it requires the correct graphics card, the correct driver, the correct resolution, the correct player with the correct settings. etc. From where I sit, either the new Theatertek (1.5.23) with ffdshow enabled or ZoomPlayer with Sonic filters and post-processing filter settings are the top of the heap. The best for plug n play is Theatertek with standard default settings.