Stability is a big one for me. I also like the fact that it's fixed, so I don't need to waste time when I do a gig or play at home. It's just done. Slippage is also an issue as the screw in metal end pins doesn't always catch that well. Soundwise, I have noticed an improvement.
Finally, it makes you play about 130% faster, better and with precise intonation.*
* This may be a lie.