RE: Hitec 5645 servos, Are they good servos?
The digital circuit does make a big difference in the centering ability of this servo vs. its analog brother. In my experience they felt very similar to the analog/ coreless 945MG servo (now 985MG) in the air.
The JR4721 in certainally a valid option, but for that price you could go with the 5985MG or 5955TG which offer even higher performance.
Most people feel the 5645MG's work very well in sport/3D models, but if you are looking for an ultra precise servo then you should opt for the 59XX series.
Mike.