I've been looking at this too. Apparently from the thread over at AVsim, it is a stand alone full featured PMDG airliner, with
the same features as the much more expensive 737-700, for only $35 US. PMDG seems to be using it as a loss leader to
introduce more sim fliers to their products, who might otherwise hesitate to spend the normal amount asked.
And it's working: I don't normally fly heavy metal like this, but at this price, I'm curious enough to give it a try.
The only minor downsides from the AVsim thread are that only a passenger version is offered, no freight version (although
as someone on the thread commented, looking out the cockpit window, you can't tell what version of the 737 you're in),
and in real life I believe there were only 192 of the 737-600's produced, so not a lot of real life liveries. However, this hasn't
stopped repainters on Flightsim.to from pumping out a bunch of fictional liveries. There are a number of actual liveries included
with the product, although I don't know how many.
Anyway, here's the thread on AVsim if you want to read more:
https://www.avsim.com/forums/topic/622324-pmdg-737-600-released/
-Rob