Cam gears do add some power, however the gains from them are minimal at best. They're really a means of adjusting the power band to some extent. When mated with aftermarket cams, they are often used to help correct lopey idle associated with some aftermarket cams as well. DO you absolutely need them?? No. Are they useful? Yes?

Difference in prices among cam gears has to do with Brand, r&d involved, etc.... Is one significantly better than the other?? Don't really know. I don't think a cam gear shootout has yet to be done. However, the more popular ones tend to be AEM and HKS as they are proven parts with a lot of r&d behind them, the others I have no experience with but would imagine they would be fairly good as well.

As to tuner recomendations, Buschur is one among many. As he may not use them, others do. It's really a matter of preference. Just don't go expecting phenomenal gains with just the cam gears alone.

 

The changes in power that cams gears will make is all relative to the starting point. On the old L6(SOHC) motors I have seen folks gain ~45hp@wheels by retarding the cam timing. Aftermarket cams with different ramp rates will probably require that you adjust the cam timing to find the optimal setting. I saw on a turbo Honda that adjusting the cam timing from 0 netted only ~15hp at the topend, but gained as much as 80hp@wheels throughout the powerband on a 550hp@wheel setup. Retarding the cam timing(in/ex) can also shift the power band up. If you have modded your car as most do with intake/exhaust/turbo/cams then get the cam gears and spend an hour on the dyno finding that extra power.