As with everything, it depends. I enjoy the seat up front. The little extra cost generally means that, if someone's in that middle seat, the plane is full.
But AF/KL has pretty high load factors.
If you're flying from MAD, you're going to be making a lot of connections in CDG, and some in AMS. Those couple extra hours on a European itinerary can turn a half-day travel into a full day. How well do you work in transit? For me, I work better than in the office, so that's not a problem; if that time is time lost, however. Also, connecting, you're doubling your chances of something going wrong. If you're always connecting, then it's better to go plat than free agent.
Coming across the Atlantic, I often use the arrival layover to take a shower in the lounge.
A lot matters with your home airport. How is security there, with and without priority? Passport control? What's the lounge like?

In the end, there's no right answer.

My advice: test two intra-Europe flights, round-trip: the first in economy, as you normally would, and the second in AF/KL business. Measure/observe the difference. AF/KL platinum pretty much gets you everything but the food and service on board. And sometimes there's someone sitting next to you.

Plus, you'll get a 60 XP head start.