Well the good things are that it's prettier, honestly, it is, no more low-res icons and stuff, everything looks sharper. It is more personalizable than 8.1 where you could have only your accent color and tile picture view on the start, now that's changed, as you know you can put a whole picture on your start and also set the transparency as you wish, you can even set it up so it would look like there are icons instead of tiles.
You can also play with the resolution and scaling. In my case, with the 640, i scaled down the slider do 175% (i think) and now i have 4 columns on my start screen and everything looks so much damn sharper. (You enable this by going to Settings -> System -> Display, and there you pull down the slider to 175%, assuming you h ave already enabled 'Show more columns').
Oh, and if you've been to previous build, i have to say that Store app has become almost perfect. Downloads are superfast now and MS is working and releasing bug fixes for each phone individualy that would fix the donwload and update errors.
Speaking of settings, now it is easier than ever to surf through your settings and managing them, much more intuitive and organized.
Now the things that are broken and bug me. In my case, don't know if everyone is facing this issue, glance is not working. It was functional on previous builds and now, well, it is not. The settings for glance itself are there, but if i try to edit them the screen goes blank and i just have to exit the app.
Next, you may face some lag after upgrading, such as slow pin input on lock screen, waiting for tiles to appear, and some apps not working, all that is easy resolved with a hard reset (which i do every time after i upgrade), and it really speeds up your phone.
One thing that seriously annoys me, and there are other people reporting it, is that you're unable to connect to your PC via USB cable. It simply won't recognize your phone.
Other than that everything is cool and ok. I love the new Phone app and combined Messaging+skype app. I'm loving more than 8.1 so far, but to be honest, it's not as fast but it makes that up with it's fresh looks and revamped UI.