At four months old (for Buster) I just bit the bullet last night and upgraded my Stretch/stable to Buster/testing. I wanted to do it all correctly finally, and have my system doing Debian testing the way it was intended as it "could, and should be" done. The dist-upgrade worked fine. Nothing "broke." No debian-gotchas.
However, and perhaps a good thing, I've not seen a lot of differences between the two. Everything I could do (that I've tried so far), I still can do, and it all works as it did prior to Buster. This release is about four months old (today's Oct. 14, 2017), so perhaps it's early to expect differences between the two. IRC (petn-randall in #debian on irc.oftc.net) predicts we're 1.5 a. from Buster/stable.
On the other hand, at four mo. old, I was already trying to die from Whooping Cough, so what's it's problem? I (thankfully) failed at that (but I might have helped create the vaccine; woohoo!).
So, is anything in Buster "new" with respect to Stretch, aside from fixed bugs and security atrociousness? I suspect there's lots of changes between Stretch and Buster in the system's server daemons, but far less noticeable from the user interface point of view. I guess the devs may weigh in here sometime, when they understand what upstream's been up to lately.
Then again, debian-goodies (which provides dpigs) is new to me; never heard of it before! My smallest dpig is firefox-esr!
FYI, I use Mate desktop lately, so Cinnamon, KDE, Enlightenment, fvwm, ... users, take my UI comments with a grain of salt. I don't think this stuff is UI dependant, but correct me when/if I'm mistaken, please and thanks.
I thought Stretch the best Linux and Debian I've ever used. Now Buster inherits that. I love the ride.
I probably ought to mumble something about Skynet here now ...