CNET did not subject the Panasonic TC-PS64 series to a full review, but we have had extensive hands-on time with the 65-inch version. We've also reviewed the closely related TC-PST60 and TC-PS60 series.
Based on our observations, the S64 is a sort of hybrid between those two series. It offers the same screen filter technology as the ST60, which means it performs the same in bright environments. That advantage makes it a better all-around performer than the S60, which suffers under the lights.
In every other way it's essentially an S60: the same deep black levels and relatively accurate color (but not as accurate as the ST60's), the same inability to properly handle, the same excellent input lag score (35.3ms). The picture adjustments are the same as on the S60, as is the rest of the feature set: bare-bones Smart TV apps, just two HDMI inputs, and no 3D.
If you can afford an ST60 it's probably worth paying extra money for its advantages, including slightly better picture quality (namely color and 1080p/24 processing). That said, if I did a review of the S64 it would probably get a "9" in picture quality--the same as the ST60, and one point better than the S60--and a "10" in value, making it one of the best TVs of 2013.
So yes, the easiest way to think of the S64 is as a "poor man's ST60." For gamers sensitive to input lag, and people who don't consider the ST60's advantages worth the extra money, the S64 is a tremendous bargain.
The main downside is that the S64 is only available in 50- and 65-inch sizes, and only from wholesale clubs like Sam's Club and Costco. As of October 2013 inventory is relatively low on these models, according to Panasonic, and the company couldn't tell us whether it would be replenished in time for the holidays.