Pro:
- Faster
More memory available for your use
Some broken things work (bluetooth!)
more stable
no junk-ware
Con:
- voids warranty
No support - if you need hand holding, your carrier will be of little help
accident's happen - you might brick your phone (rarely permanently)
The bottom line is, if you're happy with what you have, stick with it. Most everyone here has had one problem or another that custom ROMs help with or fix - and they're willing to take all responsiblity when it goes wrong upon themselves instead of calling their carrier.