A 650 or 651 in good condition can sound very sweet. Not 100% accurate theatre organ, of course, but very nice. The 651 has a few extra bells and whistles that make it worth seeking out.
The 652 is a different animal entirely. Some say it lost the sound of the original 65x models, and I'd agree to a point. But I really did like this organ. There was a good voiced percussion section that 'floated' between manuals. A transposer too. It could make a darned good sound and I used it for a few BBC sessions - I'll have to see if I can find the tapes, but they were in mono back then. My little backwater of the BBC hadn't got much stereo recording gear at that time.
The 652 does have some reliability issues, mainly with the LSI technology used for the percussion voices and for the reeds and brass on the third manual.
As for the 653, this is a bastardised (excuse the language, but it's the correct term!) instrument, made after Kimball took over Conn. The spec was good but I didn't like the sound and some of the gimmicks put onto it. I'm told, and read on the Net,that it wasn't that reliable, but there aren't enough in the UK for me to make that determination.
And there's always the gorgeous 580 3-manual spinet, a lovely sounding organ.
A 650/651/652 would be a good organ for building skills, and they can be MIDIfied so you can use them to drive something like Miditzer, Hauptwerk or J-Organ.