We embraced it wholeheartedly when it came out in 1985, and it completely changed the shape of the game, IMO for the worse. Gradually we dropped more and more and eventually went wholly fundamentalist and declared the entire book off-limits and stuck solely with the PH & DMG rules. In later years I've relented a bit. There are a couple things I use in all my 1E games (alternate unarmed combat rules, barding rules, spellbook rules, some of the magic items), a few things that aren't "default" available but that I'd probably allow if a player requested it (wood & wild elves, new weapons, new spells, armor-based ability adjustments for thieves, thief-acrobat class), plus I've got a soft spot for the barbarian class (which is of course grossly overpowered and not really suitable for a traditional group-based party at all (since they overshadow fighters and thieves and hate clerics and magic-users) but can be really fun for a one or two player sub-campaign). Most of the stuff that actually changes the shape of the game, though -- Method V stat generation, "super-races" that are unambiguously better than the standard PH races, greatly increasing class options and level limits for demi-humans, allowing thieves to use bows and m-us to use slings, weapon specialization, making the paladin into a subclass of the cavalier, "super-armor" that absorbs damage, etc. -- I don't allow at all. If I were to allow all this stuff I think I'd rather just go all the way and play Mythus instead (IMO UA is better seen as transitional between AD&D and Mythus moreso than straight AD&D).