The Uni-ball Gel Impact rollerball pen (UM-153S) is a nice pen to sketch with for those who prefer broader strokes.
Depending on which country you buy from, the pen may be retractable or is sold with a cap. Mine's bought in Singapore and comes with a cap. Personally, I would prefer the retractable one as it's more convenient.
It has a rollerball head of 1mm and produces a 0.8mm line. The stroke is quite broad hence in order to put details you really need to draw big. Personally, I prefer using this with paper sizes A4 and above.
The rubber grip is quite comfortable to hold for long periods or writing or drawing.
Design of the pen is simple with a transparent portion on the pen's body to check the ink level.
The gel ink is extremely smooth on paper. It just glides along. You can get UMR-10 refills when the ink is used up which is very quickly because it deposits a lot of ink.
Three colours are available: black, red and blue. The black ink is waterproof and fade-proof so it's a great pen to work with for watercolour or mixed media. You can spray fixative on it and the ink will not break apart.
The pen is prone to leaving a blob at the end of the stroke. If you leave the tip on the paper for long, the ink will continue to flow out a bit and then stops, so you also get a blob that way.
The ink does not dry that fast so be careful of smearing, or when you're mixing media.
Using watercolour over the waterproof ink is fine.
For some reason, this pen is more expensive than the popular Uni-ball Eye (or Vision) (in packs of 12) and a few times more than Signo Gelstick Ink Pens, another gel pen that I really like. However, those pens that I mentioned do not produce such thick strokes so it's not a fair comparison.
In terms of value, this pen can be pricey, and is made more so as the ink gets used up pretty quickly. However, the performance is quite good even though there are signs of creating blobs.