I have used the DVDXCopy products since they debuted - XCopy, Xpress, Platinum - you name it. But now I only use DVDShrink and DVDFab. DVDShrink has a "deep analysis" feature that allows dynamic compression based on frame-by-frame content. Also, DVDXCopy puts on a nag screen that stays up for 20 seconds or so saying that the disc is a backup.
I did my own test where I compressed a DVD using DVDShrink and another using DVDXCopy and wrote each to a DVD+RW. I labeled the discs, put them in my carousel multi-disc player without looking at which was where, spun the carousel a few times and tried to see if I could tell a difference between the discs w.r.t. picture quality. I could tell a distinct difference at 70% compression even on my 22" non-HD TV and the superior picture each time was from the DVDShrink disc. For those curious, I used Infoedit and VOBrator to get rid of DVDXCopy's nag screen such that it wasn't obvious when I played them which program made the disc.
But the main reason that I will never go back to DVDXCopy is that they put a unique identifier on each backup DVD that identifies the user. I found it offensive that I was one of the first users who supported the company and did not know of it. Perhaps it was mentioned in the license small print.