The Istar 5" f12 is an excellent lens in my experience. Please refer to the link below for more details on my experience.
In comparison to a regular 5" f12 achromat lens in cell, the Istar is comparatively heavy due to the thicker glass elements. The reduction in CA is a benefit however if I was doing it again I would choose the standard Istar 6" f10 lens in cell as it weighs the same and has greater aperture and costs almost the same price as the 5" f12 R30 lens.
The only advantage to the 5" f12 R30 is the reduced CA on planets and bright stars and it is a personal choice whether that is preferable to greater light grasp with some CA. A standard 6" f10 or f12 can always be stopped down to 5" or less to reduce the CA for planets but will still be far superior on DSO's where the CA is not obvious.
https://www.cloudynights.com/topic/3.../#entry4171500
Andy