OK, I did decide to go into town, and I'm glad I did, but I don't think staying would have been that bad. I know md says it constantly, but the airport bus was certainly convenient, inexpensive and relatively quick (relatively little traffic on a Sunday).
And the weather was great for strolling around. I went to Jing'An, walked to Madison for brunch, and then headed back, quick walk around the temple area (but didn't go in) and then hopped back onto the bus to the airport.
If this was an only visit to SH, and one had a long layover, would definitely recommend it. However, the travel to and fro ate up 2.5 hours of my 5 hrs, and of course, I wanted to head back a little early too..
If the weather was bad, or I had more baggage, and if like me, you are happy either reading a book or catching up on work, I would consider just staying in the airport. The free wifi is unlimited (unlike some airports) and the seats are comfortable, and for domestic arrivals, you stay airside, so have access to a lot of shops/ restaurants etc.
tb