Seat assignments: no benefits at all. You still have to pay for preferred seats, otherwise you are limited to the same general seat selection as all other fliers. Gate agents/check in agents may have some lee way but nothing is guaranteed. The system will also auto assign you like general pax (including middle seats) you must must must check with an agent for preferred seating.

Lounge access: you get access to the ever-so-crowded business class lounge at PVG. It's definitely above average for China, in my opinion, but may fall short of your expectations if you compare it to DL Sky Clubs or the nicer AF lounges. Only China Eastern Platinums get access to the First Class lounge section. When at out-stations, be careful: if you are flying economy and hold status, they will by default give you the worst lounge. Business/First pax get the invitation to the nicer lounge. For example, at JFK you will be given an invitation to the Alitalia lounge, despite being able to access both Korean and Air France as STE+. (Which is not saying much, since both of those lounges are crappy Priority Pass lounges, but still better than Alitalia.)

Free/op upgrades: Don't count on it. It rarely, if ever, happens. You can purchase it at the airport, but I don't know the cost. It's also possible to buy on board via the IFE, but the price was outrageous.