Hi, I am thinking about buying some Fast Frames for my Janome MB4. I was wondering if anyone has a set, and if you use them. They are quite expensive, and I was wanting to know if they are worth the cost.

I don't know the fast frames, but I have the Quick Snap frames by DZGNS for my Melco EP4 which is equal to the Janome MB4.
They are expensive (especially if you have to pay import duty and freight to Europe). I like them for small projects. Hooping is easy, but placing the fabric right is trickier than in a normal hoop as there are no guide lines on the frame (so I made marks on the upper frames). Disadvantage: the frame is held only at the middle of the top, not on two sides, which makes it less sturdy when using the bigger sizes and/or heavy fabrics.
To be honest: I used the frames a lot at first, now I only tend to use them for things like pant legs, sleeves, tote pockets etc.