Many, many times. I would recommend using them opportunistically: plan your trip based on which flights have space.

I used D3s for years while in college, and I believe the best uses are for Europe, 3-cabin transcons, and South America. All others I would play the credit card game.*
You might even want to do that for the return: Once you've locked in Flagship First on JFK-LHR, you might want to book a return home as a backup, just remember to never hold space on a flight you might list for.

*I say this because people often save their upgrAAdes for Asia, which can fill up J and F and make them less predictable. Plus, mileage redemption rates there tend to be better. Domestic flights are very unpredictable and can fill up very quickly, and be booked solid for days. It's also nearly impossible to get in First, other than on the transcons where the business cabin acts as an 'upgrade barrier'.