Yes, we used it for the most part. The increased level caps were welcomed as I recall. We used the spells pretty extensively, as well as many of the magic items (which all made the change and lasted well into 2e). We didn't use the new races so much, but not out of anything other than circumstance - nobody was interested in playing the new variants. We also used the new classes pretty extensively. I recall the cavalier in particular as being heavily unbalanced. The barbarian was a bit better, assuming that your DM enforced the class' restrictions. Didn't UA introduce expanded specialisation rules as well? We used those too. My main problem with the book, actually, was that it wasn't well incorporated into subsequent adventure releases (I recall seeing some implementation of its rules here and there, but nothing widespread). This made UA characters more powerful than their levels would suggest when running them through 1e adventures, as those adventures weren't generally tailored for UA characters.