We have now converted about a dozen BMW E46 M3 CSL models to a manual gearbox and it's fair to say the benefits of the conversion have been well received.
The conversion is converted with OEM components and can be reversed back to SMG very easily.
The reality is that once you have experienced the manual conversion and addressed just about the only fault associated with the thrilling CSL driving experience - you are unlikely to want to go back!
The video on the link below by Henry Catchpole at Carfection describes the experience better than we could ourselves!