Hello aardee, welcome to beautiful Brussels!
Yes, I think M&M is a good programme to join if you fly business class. Another one to consider is bmi Diamond Club (although this would probably be changed into M&M as well at some point in the future). If you achieve SEN status, you can also use the Brussels Airlines lounges in Brussels.
And at some point this year or early next year, I would expect Brussels Airlines to be a M&M airline as well.
If you decide to sign up, let me know, and we can both earn some miles if I recommend you.