I bought the Yamaha DVS5750 at Best Buy about a month ago. Seems like a good machine. I haven't seen any reviews out there, so I'll be looking for any responses you get. My own feeling is the player's quality is good, the remote could be a little better for a player at this price. (I got it for $150.)
I have had the DV-S5750 for three weeks now. I could not find many reviews. There was one British mag that gave it 5 stars and one that gave it 3. The mag that gave it a 5 gave the HK 22 a 3 and visa versa. Even the 3 star mag loved the CD audio performance. I guess it is all in what you like. Audioholics.com just reviewed the C-750 in detail and it appears to have the same video processing and 2 channel analog audio DAC's. They thought the C-750 was a great value and loved the the CD audio performance.
Yamaha appears to be getting their new line of lower end DVD's from Philips. I am not sure if they specify any different internal components, but again in the British mags, the Philips units with the 12bit/108mhz video DAC have gotten good reviews.
The bottom line is how does it look and sound to you. Try all of the units you are interested in if you can. I have a Mitsubishi WD-52525 DLP with HDMI input. I debated getting a unit with HDMI or DVI out and upscaling. It appears that anything under $250 to $300 is junk and the actual visual improvement is up for debate. There appears to be some nice gear at $600 to $700, but that is a lot of money and you still do not have a true HD player.
To my untrained eye, the video from the 5750 is very good. Not as good as a high quality HDTV broadcast, but pretty close, if not equal to an average one. The question is how much do you want to spend and is the difference if you can see it worth it. For my money I am going with the $150.00 unit ($135.00 after crediting back the coupon I got after the purchase)and wait for true HD which should be beyond belief or more reasonably priced upscalers that really work.
Hope this helps. Let me know your experience if you go with the 5750.