I think it is overall worth it, mostly for the reasons that SEA_Tigger just stated, particularly if it's one of the longer transatlantic flights such as SFO-CDG or ORD-FRA.

The only way I can "enjoy" (cough) economy class on transatlantic flights is in seats 17ABHJ. Plenty of legroom, and enough space to set my carryon down and use it as a legrest. In fact, if you only have enough to upgrade one way, I would recommended upgrading on the overnight portion (US -> Europe) and then try to get 17ABHJ on a return daylight flight.

Just my two cents!