Are they all rubbish or are some of them worth buying?
If I assume you have all the albums including "Live At Brixton" I can give the following advice...
| Year | Compilation | Buy? |
|---|---|---|
| 1998 | Who Cares a Lot? (2 CD version) | Yes |
| 2003 | This Is It: The Best of Faith No More | Maybe |
| 2005 | Epic and Other Hits | No |
| 2006 | The Platinum Collection | No |
| 2008 | The Works | Maybe |
| 2009 | The Very Best Definitive Ultimate Greatest Hits Collection | Yes |
Maybe: Both Maybes are for the track "The Perfect Crime", previously only available on the "Bill & Ted's Bogus Journey" soundtrack. These compilation would be much easier to find these days, but the soundtrack can still be found for cheap on eBay if you'd prefer to hear it in context.
No: These CDs have nothing extra on them that isn't on the albums. The only exception might be that you cannot find a copy of "We Care A Lot" (their first album) although buying that album on CD would be much preferable to these compilations.
Yes: Who Cares a Lot?: Previous to the 2009 compilation this was the only place to get previously unreleased tracks The World Is Yours, Hippie Jam Song, Instrumental, I Won't Forget You and the demo version of Introduce Yourself. They also nicely included live versions of Highway Star, Midnight Cowboy and This Guy's In Love With You. The main compilation also includes I Started A Joke which was a single at the time and previously only available as a b-side in 1995. Post the 2009 compilation the only reason to get this collection is for the Introduce Yourself demo and Instrumental, neither of which are particularly exciting to be brutally honest. Oh... and I Started A Joke.
Yes: The Very Best Definitive...: The second disc of this compilation collects most of Faith No More's original (non-cover) b-sides and also adds New Improved Song and Sweet Emotion, available for the first time on CD (previously only found on rare magazine cover vinyl singles). These would have been enough to recommend this CD, but on top of that, the version of Sweet Emotion on this CD is longer than the vinyl version (which faded out).
But: If you're one of those people who hate modern remastering (which turns everything up to ten thousand) you'll hate the new compilation. You may just have to go dig out all the singles which include the b-sides you want. Even then, the 2009 compilation is currently the only place you can get the complete Sweet Emotion.
Also, the compilations by themselves still miss many many quality Faith No More rarities. I suggest you read my Faith No More collector's guide.