I bought one, it got killed by my cats, so I bought another, it got stolen, so I bought another. The cat-killed one got fixed, so we and my Missus have one each. She spends hours on Lumines, whereas I use mine for downloaded TV shows on the way to work. Oh, and Everybody's Golf, GTA3, Ridge Racer, Wipeout ...
Obviously, I'm going to say they're good, but if you get one, get a 1GB memory card. you can fit a movie on 600-700MB in MPEG4 (h.264/aac). I'd not bother with buying movies on UMD unless you can afford it. I prefer to buy on DVD and convert.