I've heard something about Twixtor or then . . . retiming the native 30p footage.

Not with the best results, though. For reasons of quality or because it's not seen as convenient.

And about Cineform? What can we expect from Cineform?

Is there any reliable Cineform software solution for the conversion of 30p to 24p or 25p?

Is it efficient?

30p to 24p is really not all that practical. It is either extremely compute intensive, with so-so results, some footage will look fine, other footage less so. CineForm does not use compute intensive techniques.

I'd be staying at native 30fps myself, but the best frame rate conversion software I've seen is MVTools - available for free as an Avisynth plug-in. It's dog-slow with 1080p video but the results are very good. I used it to resample 1080p60 video to 720p25 for use on streaming HD site Daily Motion.

Although 60p to 25p is easier than 30p to 24/25, which has larger motion steps to interpolate.