More than just a smaller version of the Vac 700, the Vac Mini is a more conventionally-styled pen, by TWSBI standards. It's small, but comfortable to use - for a full-length pen, the cap can screw onto the back of the barrel, but it's long enough for most to use unposted, too.
The vacuum filling mechanism makes this the perfect pen for pen geeks, but may seem odd to those without so much interest in fountain pens. It can fill itself very full, especially with the Vac 20A ink bottle, and has an impressive capacity for a small pen, holding much more ink than most full-sized pens can.
The blind cap, when fully screwed down, closes a valve, preventing ink from flowing. This may not seem useful at first, but it keeps the pen sealed so only a small amount could leak out, even if the cap gets knocked off, and reduces the chance of spilling ink into the cap if it's knocked about in your bag or pocket. It should make it safer to fly with, too, though TWSBI don't make any guarantees there. For short notes, you can leave the valve closed, but for anything more, you'll need to unscrew the blind cap, and release the ink - just don't pull it up too far, or you'll make a mess!
If the unusual filling mechanism isn't for you, but you like the smaller size, see the Diamond Mini - a smaller version of TWSBI's Diamond 580 piston filler.