I've used one of these for over a year, for notes and underlining in books. The ink flow is good, does not clog, and the pen itself is relatively light. The mechanism works ok and the barrel has a smooth rubber grip which is (for me) comfortable to hold. The clip is useful if you want to secure it to a notebook cover or pocket.
For years I bought the Bic Four-colour one but after a few months the ink in those goes sludgy, especially the green ink. The Zebra ink doesn't do that. It remains free-flowing, hence I've just ordered another two Zebras.
(The body colour is black plastic, which is easily mislaid, so one possible improvement would be a bright coloured strip around the middle to make it easier to spot, or you can do this with a strip of bright tape.)
To sum up, a reliable tool for four-colour notes or sketches.