Uniball Signo DX 0.38 is been one of my personal favourites. I have been using it since young when my mother introduced me to this pen. It has a nice professional design. The see through body tube shows the ink level in it. Rubber grip provides an adequate hold, and this pen can last on hold for a few hours without feeling discomfort.
Huge plus is its thin lines. The blobs it makes aren’t obvious. Ink flow is great. It says on the label that it is waterproof. Indeed it is. Here are a few sketches I made.
These are my old sketches, earlier this year.
These are the sketches I made at the zoo yesterday. It shows how great it can be used as a fast sketching instrument.
It produces dark lines but not as dark as other pens. However, it all changes when you drop and damage the tip. The ink flow will be impaired and cannot be fixed. You will have to buy a refill. Handle with care and make sure to cap the pen if you are not using it, in case it drops onto the floor.
I highly recommend this pen as a sketching instrument. I think the price is reasonable.