This review is by Kalina Wilson, who can also be found at geminica.com.
The Platinum Pocket Brush Pen is in the family of brush pens that aren't actually brush pens. Instead of individual bristles, these pens have a single molded tip that mimics the silouette of a brush but doesn't fan out against the paper the way a true brush would. Something about this design makes for a very diverse assortment of products! These false brush tips can be wet or dry, sturdy or fragile, floppy or stiff - it's a minefield and a treasure trove out there, with each pen feeling very different from the next when you get it in your hand.
The Platinum brush stands out visually due to the gold flowers stamped onto the barrel and a sparkly gray clip (that likes to come off in your hand when you try to remove the cap). It looks and feels a bit cheap - but it is cheap, at $3.
Among the various brush pen options, the Platinum Carbon does have a unique set of characteristics: it is waterproof, fairly dark, and has decent flow. The other options are either not waterproof (the Uniball and Pilot brushes), don't provide a large wet line (Tombow), or aren't as dark or smooth (Copic).
The nearest equivalent to this Platinum Pen is the Copic. If you like Copic brushes in general, I'd say there's not a lot of cause to try the Platinum unless the slightly darker ink is a big advantage to you. Personally I haven't had a lot of luck with Copic brush lines, and actually prefer the Platinum brush to work with even though it's cheap and kind of stubby-tipped.
I won't lie to you - I don't love this pen. I prefer to be able to get a finer line, and if I'm going bold I prefer the feel of the Uniball brush even though it's not waterproof. However, if you are looking for a moderately wet-writing disposable pen with a waterproof line for the purpose of doing bold sketching, this Platinum brush is certainly worth $3 to try!