It's been about two months since I first starting using my VCR with my DTV coupon box. The VCR is a JVC HR-S3900 (bottom line model) with a CM7000 outputting S-video to the recorder. Every day I tape at least 1.5 hours of digital television. I have experienced a few problems where the signal drops-out such that the tape records "signal lost". This is a flaw with 50-mile long-distance reception, not the VCR.
Super VHS mode - The image quality is indistinguishable from the live image. In SP mode there's no visible loss, and the image looks as clear as when I'm watching a DVD. In the slow SLP/EP mode there's a slight blurring on very small print.
VHS mode - Due to horizontal resolution reduction from 704 to 370 pixels, the recorded image has a definite blur, but still looks better than a store-bought movie. And of course it looks better than the old analog signals.
DVR ReplayTV - I also tested my DVR, but the results were less than satisfactory. In nearly-all the modes the Coupon Box image was degraded with lots of macroblocking and pixelation. Based on these results I may sell my DVR.
Internet - I find myself doing a lot less taping than I used to. More-and-more I watch videos downloaded directly off the web, rather than set my recorder. The quality is poor, but the instant access is more convenient.