I just bought a BDP-S1. I have to admit that I'm pretty disappointed so far. The build quality is ok, but much inferior to my Onkyo DV-SP1000 player.
Here are some things I didn't like right away (in no particular order):
1) There's often a long lag between the time a button on the remote or the unit itself is pressed, and the time the unit responds. I find it annoying, and I often try to press again, which may reverse the intended operation.
2) The power and eject buttons on the unit provide little tactile feedback, and this coupled with the lag problem is quite confusing - you simply don't know if you pressed the button or not.
3) The setup menus get buggy sometimes and some of the menu choices appear blank.
4) When a disc is ejected and re-inserted, the BDP-S1 doesn't resume play like the Onkyo does.
It feels like this product was rushed out.
I haven't done much viewing yet. The image quality with regular DVDs is very good. From the little I've seen so far, it seems pretty comparable to that of the DV-SP1000. I was very disappointed with the bundled Fifth Element disc. I compared it to the standard DVD of that movie, and the image quality provided by the Blu-ray disc is only marginally better. This is on my 50" 720p Panasonic plasma. I hope other Blu-ray disks are much better than their corresponding standard DVDs. If Fifth Element is an exception which is not much better than the standard disc, then they really shouldn't have chosen it for bundling with the unit. What were they thinking? Some people may return the unit just because of this bad first impression...
I'm not all that thrilled that I bought this machine at this point. I'll watch some more Blu-ray movies and see if my opinion changes. Right now it feels like I should have waited for a next-generation player without the kinks, and with the new stuff that's coming (BD-R/RE, HDMI 1.3, audio codecs, etc.)