The side art with the LEDs are incredible. Definitely pay for the extra sticker for the top to cover the LEDs, or anyone watching will be blinded by the LEDs.
Color DMD is great.
Snake fangs and tongue sticker are no brainers.
Shaker is a must.
If you want to spend the money, the purple armor (including coin door) from Pinball Refinery looks REALLY nice.
Getting Sparky custom painted really makes the machine look better.
Backbox, speaker, and under cab lighting really makes the whole experience better.
Upgraded speakers are nice. Don't expect crazy bass, as it just isn't in the mix.