It looks like we're talking about a few different A/C compressors in this thread; I thought the OP was asking about the Old Air Pro6TEN Compressors available from Ames and elsewhere, which are claimed to be an improved, aluminum version of the original Harrison A-6 compressor that most of our cars came with. I know that 5th T/A commented on his experience with the original A-6 compressors (not good) and 75taguy mentioned that he replaced his A-6 with a new aluminum housing compressor, but it isn't clear what brand or design. olds403 mentioned the Sanden FLX7, which is another design of compressor.
What isn't clear to me (and I would like to know) is whether anyone has any experience with the Old Air Pro6TEN Compressors. If they work well, it looks to me as though they could be painted black and stickered to look like a Harrison, but could be more reliable and weigh HALF what the OEM A-6 does - and they weigh a lot. See attached photo from the Ames catalog: