Thanks for the replies guy, yes it's certainly a weird one.
I certainly wouldn't trust my movie collection to Princo.
Yep I agree totally. That's the thing that was so frustrating about this problem. It made it seem like Princo was somehow the only media that was fully compatible with my player, so even though I didn't like it I kept on using it. It wasn't until I eventually started finalizing all my discs that I realized other -R media also worked good, just as long as it was finalized. I know I should have been finalizing my discs all along but since it didn't seem to matter with Princo I naturally assumed it wouldn't matter with other -R media either, that's where I go misled.
I'd try DVD+R with and without booktype set to ROM.
Yes that also plays ok. For a long time however I didn't know about booktype setting and this particular player wont play +R media unless the booktype is altered. It's only very recently (after flashing my Pioneer 108 to PIOdata firmware) that I can actually use DVD+R media with this player.
You don't have to waste very many discs to make a new player look like a good deal.
Yeah they're certainly cheap enough now. Still this player is ok now I've worked out this quirk. Like you said the discs should rightly be closed in any case, and it's no hardship to do so. I think you can see how I was led astray by this strange player behavior - when I first started out I was using Princos (as many newbies do until they learn better) and there I found that finalizing seemed un-important on my player and so got into the bad habit of not bothering to do it.
Of course the other embarrassing side effect of this quirky behavior is to think of all the times that it led me to tell people that "Princo are the best media" as they appeared to have the best compatibility on my home player. Ouch!