Question: XCode 6 Beta 5 and autocomplete stopped working. I thought problem in Beta 5 but I've reinstall to Beta 4 and it's still not work but it did before. I check preferences for "suggests" and I tried to delete Derived Data but it doesn't help

Answer: 
1. Clear derived data.
2. Under build settings, select Always Search User Paths to "YES".
3. Clean the project.
This happened to me recently because I had a /\* \*/ style comment after the @end of my @implementation.

Not sure why that would break autocomplete but moving the comment into the implementation fixed autocomplete.

There seems to be a lot of reasons autocomplete breaks. Just adding to the list..