Having now played with it for a day or so... I do really like it. It's a lot more clear on exactly how many more games you can play... and I do like that the more you play you're no longer facing diminishing returns. Even though I'm now playing both Wild and Modern, I really don't think I'll play more than 50 games a day, so I don't really see myself spending any DEC to increase my energy. I do like that this new energy system encourages a good win/loss ratio over just playing thousands of games.
Overall I think it's a good change. I'd be curious to see how it affects the ecosystem overall.