Writing wise, I found the Plumix didn't write as well as the Varsity. It seems to scratch the paper whereas the Varsity seems to glide across it. Like the Varsity, the ink does bleed through the paper you write on. This can be a problem if you try to write on both sides of a sheet of paper. At times, the pen would poke through the paper where I'd written on the other side with it. The pen uses a standard medium stainless steel (I believe that's what it's made of) nib.
The pen comes in single pen and 3 pen packs. It has 3 colors - a translucent purple, translucent blue and translucent black. This can be handy as it enables you to see if you have a cartridge in the pen. I don't know if you can use a refillable ink cartridge with the pen.
Overall, it's a decent cheap fountain pen though I'd recommend the disposable Varsity as it seems to write better. It does have the advantage over the Varsity in that it's refillable and, you can better see just how much ink you have left in the cartridge. Varsity has a window where you can see this somewhat but, I don't think it's as good as being able to look directly at the cartridge. Another problem you may encounter with the Plumix is being able to buy one. Fountain pens aren't easy to find if they can be found at all. Pilot does sell the pen on its website http://www.pilotpen.us/ which can be the easiest way to buy the Plumix or Varsity. Happy writing all.