A quick calculation to see whether paying for the booster is worth it.
For me most value in using the MR points is converting them to Avios.
That's a 3:2 conversion.
If you spend 385€ on the card you will earn 1155 extra points thanks to the booster (multiplied by 3 as you would get 1 point without the booster).
1155 MR points are the equivalent of 770 Avios.
I value Avios at about 1.3 cent so 770 avios are worth 10€.

Means you need to spend 385€/month to get breakeven with the booster, everything you spend on top of that makes the booster worth it.

You mileage may vary, if you can get more value out of your MR points you will need to spend less.
Happy to hear how much value others get out of their points.