RichGreen said:
I bought Unearthed Arcana when it first came out but haven't really used it. There have been several threads about "most useful" books and UA comes up time and time again. What bits do you like and use in your games?
As a general suggestion, don't overwhelm your game with variants, you have time to try more later in other campaigns

Start with trying out the variants which seems easier to add, and which don't interact with each other:
For example you can allow a lot of the character variants (not the generic classes or gestalt, these are variants which should be used for all PCs or none), even all of them, and see what your players pick up. They are almost all well balanced.
Some mechanics variants or add-ons are very easy. You can consider e.g. the following:
- Contacts: these are very useful, we have loved them so far
- Weapon Groups: make the PCs less versatile at first, but better on the long run
- Metamagic components: quite expensive but interesting
- Reducing LA, Magic Rating, Class Defense... all of which lessen some PC penalties