I don't think it is good value for money as the upgrade fee doesn't seem to buy you that much more than those sitting at the back of the plane.
PE is an economy service with an extra 7 inches of leg room, although I don't think the seat recline is much to write home about. On the Orlando route the PE seats are all on the upper deck of the plane, which is nice and quiet, and there is a better Flight Attendant to passenger ratio so you get a bit more attention from the staff. At the airport there is a dedicated check-in desk for PE customers, which might save you a bit of time if there is a queue at the economy check-in (there is not a dedicated PE desk at Virgin's Downtown Disney check-in, and it is unnecessary if you plan to use Gatwick's Twilight check-in). You are not entitled to use Virgin's business class lounge with a PE ticket. You are allowed to get off the plane before economy passengers, and in theory your luggage should arrive on the carousel before them as well (in my experience this can be a bit unreliable).
As we have to travel during the summer holidays the upgrade fee for PE always seems to be around the £350 mark (on top of an already expensive economy price). What price are Virgin quoting you to upgrade your flight next May? Only you can decide whether the enhancements to the economy service you get with PE is worth the cost of the upgrade.
Regards
Rob