I had an MC-3+ Smart Clock USB. The REF10 is more than I was willing to pay for something that I might or might not consider an improvement. My conclusion was that differences caused by reclocking will be system-specific (a pretty bland and safe conclusion).
Those interested in the subject might look at this 2010 comparison of clocks in the pro sound journal Sound On Sound. It said, among other things:
There's a widespread notion that adding a high‑quality master clock to a digital system will somehow magically improve its overall performance. While that might possibly have been the case in the very early days of semi‑pro digital converters where, frankly, some of the internal digital clock designs were pretty ropey, it certainly isn't the case today. . . . Today's converter designs generally work best on their own internal clocks, and most will deliver a slightly poorer performance when clocked externally. The very best devices will show no change in performance at all, because they have superb clock-extraction circuitry that can remove all traces of clock jitter and other external clocking artifacts, so they work just as well as when running on their internal clock.
As always, the questions are: Is there a difference? Is the difference an improvement? Is the improvement worth the price?