Mechanically, both pens worked as advertised. I especially liked the feel of the additional weight of the Mont Blanc refill. Initially, both pens worked well and wrote smoothly, and I really liked the color of the blue refill. The medium blue refill was actually a little too wet for my liking; it would bleed through my standard office paper. Both pens performed in an awesome manner for a couple of weeks.
Why am I using the past tense here? Past tense is appropriate because both refills are no longer with me - they both started to dry out and skip on me after about two weeks. I admit that I didn't use the pens on a daily basis, or even regularly. That may be why they both started to dry out. I don't own a real Mont Blanc pen, but I suspect that they are capped to prevent the rollerball refills from clogging. I seriously doubt that both refills actually ran out of ink . . . my theory is that the tips just got all gunked up due to lack of use.
I enjoyed the pens while they lasted, but I won't be buying more Mont Blanc refills unless I can use them in a capped pen. I'm not certain that they were designed to be used in a retractable pen that leaves the tip open to the air.
All of that said, if you only use one pen at a time on a regular basis, then the Mont Blanc + G2 combo might work well for you. If not, then proceed with caution!