This feels amazing. Very comfy and an improvement on the sad plastic holder I was using. As a warning though, the rose nib is too big for the small holding ring and not big enough for the large ring (it falls off). Any other nib I tried stays on with no problems. Also comes with a cap which I like cause I have stabbed my finger reaching for my other pen XD.
Other than the obvious fact that this nib holder is compatible with two nib sizes, one other aspect really stood out. The cap at the back can be used as a counterbalance. This I found to be very useful for inking smaller, more detailed areas. I wouldn't necessarily call this a con but it kinda bothered me. When I ink, I tend to rotate my nib holder slightly when I need to. The grip prevents this rotatory motion slightly but it's not too big of a deal. I previously used Speedball's holders. Even without the cap, this holder is still a bit heavier than Speedball's (smaller and bigger ones). Overall, I love it and it's a nice buy.
This pen-holder is more comfortable in the hand than standard holders with cork grips. Its rubber socket holds nibs much more securely than the metal-flapped socket on standard pen-holders. Unlike standard holders, it holds large & small nibs.
The cap is a clever addition. The compact size makes it small enough to carry, nibbed, in an empty TWSBI box.
