The mods (I know of!) on my bike are:
- Fenda extender
- Bellypan
- Rear hugger
- mini indicators (front)
- Stainless downpipes
The Fenda extenda keeps the pipes much cleaner, and the rear hugger protects the chain, shock and underside of the seat. I'm not sure how effective the bellypan is at keeping the pipes cleaner, but it makes the bike look much bigger at the bottom, makes it appear bigger. I also like the mini indicators, as I find the full ones too big on the front.
For functionality, hugger and fenda extenda are better. For the look, the bellypan is a must. But get one with sharpish angles, if not it looks a bit daft.