I've got the first part/core downloaded and installed. Seems pretty nice. While most of the EZX kits when loaded get generic graphics/UI (but better than SD2), they have a bunch of special touches for electronic kits and even the flat GUI of Number 1 EZX shows up inside SD3. All the better features from EZDrummer2 seem to be there too.
Just loading a bunch of kits and playing some of the MIDI grooves isn't really enough testing depth, how well it works is much more a long term kind of thing. At first glance so far I'm pretty happy with what they did. The new kits seem to eat a lot of ram (even without the extra mics). The larger UI scaling and resizing is very nice on my 4k screen. I don't think laptop users are going to be all that thrilled with the increased memory, disc, and resolution it seems to like though.
Downloads seem to be getting a little slower, but still going fast enough I'll have it all before dinner.