Just tried it out for the first time on today's upload. I love it. I'm not very graphically talented and so my "end screens" were just a shoddy photoshop subscribe button coming on screen, and then a thumbnail, and some ugly text in impact. They didn't look very professional or nice - but now with the new end cards it looks so much nicer, and it's also a lot easier to put together. This is going to save me a ton of time overall.
I suspect this'll also help me sell more copies of my e-book and audio course, as it's much nicer and more visible to have it on the end screen like that then to just have an interactive card coming up.
Also these actually work on mobile unlike all the annotation based end screens, so, more click-throughs from mobile viewers (who make up the majority of pretty much everyone's viewer base)
Best change YouTube has implemented in a long time!