depends if you are talking 2nd cats or primary. if you want a 2nd cat delete and a res delete (Like I have)I would highly recommend it. 100% legal, and doesnt cost much at all.
just do it and be happy. Thats all I can tell you.
However, if you are thinking primary cats, I would advise against it. State depending, but most inspections make it illegal to delete your primary cats.
2006 330i |premium package | Sports package | BMS Power Box | AA Tune | K&N Drop in | Charcoal Delete | Mforce Design Air Scoops | MMW CS Exhaust w/ second cat delete | Resonator Delete | Eibach Pro-kit | Koni Yellows | Mtec fogs | Mtec v2 Angel eyes | M3 Tri-stitched steering wheel | BMW Strut Bar | 19" Avant Garde M310's | Cooper Zeon 2xs | OEM Blacklines | WTB: Dinan High Flow Throttle Body