I have three, plus a dinky little "Elite" that came with a green faux-lizardskin notepad. I must admit to being impressed with the Montblanc refill, though not so much with its $8.99 price ...
I have several Retro 51 models, including a Diplomat FP, and a Titanium Big Shot RB, shown below. My others include a SS Tornado, and several Diplomat BPs, which I picked up over time at Tuesday Morning. Retro 51 pens are great pens for the money, but my original SS RB refill dried out before I used more than half of it writing. I replaced it with a Schmidt which also write quite well.