Lately i have been looking for a decent refillable brush pen and may have found it in the Copic Multiliner SP.
I use a sable brush for my ink work (which i still think is the best) but wanted something more convenient particularly for people sketching.
In my quest i have tried:
PITT artist pen (B brush nib)
It's not refillable but the ink is very black. the brush nib frays pretty quickly.
Pentel pocket brush.
excellent brush pen but went through the cartridges pretty fast.
Didn't work with ink.
Copic multiliner SP
Last year i bought this copic multiliner SP with 0.5 nib. This pen uses cartridges and the nibs can be swapped out. When the 0.5 nib began to wear down rather than buy another 0.5 nib i swapped in their brush nib.
Its really nice to work with and gives a lovely varied line. Its more controllable than the Pentel pocket brush. The ink is more a very dark grey but it is completely waterproof.
I still need to work some more with it but so far its my brush pen of choice.