My recommendation? Buy a large capacity Memory Stick and rip your DVDs to that. I have a few UMD movies, but mostly things that I bought as an impulse at Wal-Mart/Target when I saw them being cleared out.
UMD space in my PSP case is at a premium right now, so movies are usually the first to sit out on a road trip in favor of games...but if you don't have many games, I can see some value in it.