I do. In general, but especially if the Data is member of another struct which I'm printing, I don't want to be greeted by potentially megabytes of hex output.
Maybe a good compromise would be to show the hex representation only if it's reasonably short, but if we want to keep it simple, I'd rather have the count (although something along the lines of Data(count: 20) might read nicer than just 20 bytes).
I agree that printing just the size isn't very useful for debugging, but is great for general logging, and it's good to be conservative in these cases. However, Swift should provide built in methods for printing Data in various formats, like hex strings, without having to remember the magical formatting incantation to do so.