This a great tool to keep in your cleaning arsenal as it is great at removing dirt and grime from those tough areas to reach of your drivetrain like your cassette, derailleur pulleys and the inside of your chainrings.

First, it depends on what type of riding you are doing; where you are riding; how often you ride; how dirty the drive train/groupset of your bike is getting. Second, it depends on what tool/brush you are already using to clean out the areas Mike talks about in his post.
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If you already have a brush that fills your needs then this one could be a second or an addition. I have the Park set of brushes. Even my smalest lowest width and profile brush doesn't come close to this one. I can see where this could better get inbetween the gears of the cassette.

I use it to get the dirt out between my crankset and bottom bracket. It pools in there on some of my old commuters and it's a bit a of a pet peeve.

Street dirt and grime can mess up your chainring a bit, and it's a pain to get out, especially after rain. A stiff bristle brush like this will get rid of bits of grime, and keep your bike tip top.