Meh, average. You really have to be looking for something specific at one of these stores to want to venture all the way out to Mina. Overall atmosphere is a bit dinky, but it's useful for people furnishing a new house (appliance store, furniture store and carpet shop all in one place - as well as a supermarket).
I love coming here just for Jarir bookstore. It's not just excellent for books and art supplies, but I actually got my laptop there for a decent price - and they've got great offers on books (3 for 2, etc)
Unfortunately, the presence of the co-op makes this place way over crowded, which is strange to find so far out of the city!
I normally come here for buying large household appliances - I'm not much a fan of the other stores. I will say this about Mina centre though, they always make an effort by providing something to keep the kids entertained on the ground floor - usually a bouncing castle or like a little pool with boats. There's also a fairly decent furniture store (2XL) that regularly has sale on many items.