I was going to post a much longer thing. But reading Aurick post, now I don’t hit the button and post a 1/100 th of it. As those few lines from Aurick are much cleverer. 
Intead of doing that I am going just to add some details:
-Even if at the end, you prefer a more geared to low polygon solution, you still will have:
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Possibly the greatest 2d/3d all in one Game Interface creation. Have a look at games like age of empires, or whatever game menu, icons, bars, borders. For that , Zbrush is really useful. And quick.
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A good renderer, with control over shadwos, several lights, antialiasing, enourmouse sizes in pixels renders, fog, etc.
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Combined with its detailing-illustration abbility it can make those splash-intro screens for saved-game loading.
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The cheapest solution for 3d painting over models. I have noticed better results in high polygon models, but it could be just me. It works also for the others, but as always in 3d painters, 2d retouch needed over the texture.
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a 2d painting software with a good wacom stroke response…even in my celeron…It has tools like saturation brushes, smear, light/dark brushes, infinite material capabilities…
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Some day you art director using Max will ask you for making morph targets. Perhaps the game intro needs it for a first plane cut, were face expressions are involved.
Well, I think in some post there’s even a tut for making those morph targets (several objs in the different moment of a expression. Later on, in a very similar way, Poser or max, calculate inbetweening frames.)
…pant…and a lot more. Stop here. I will bore the whole comunity. 
So , those to say the first I can think of in the jungle of needs you’ll have for games.
And in case you don’t really like it for modelling, you will have all those 6 things for a small price.
Is an extra tool. Your base tool will be Max or Light Wave (depending on your company’s tastes) or even Milkshape ( 20 dollars, and really good for low polygon stuff ) if your company is starving
or you are just a hobbyst. If is this last situation, think of:
-Lithunwrap, Anim8or, Wings3d, Milkshape, Pixia, and some others.
Well…this time I hope I make it really clear…always my fingers do the same to me. 
However, it is not that Zbrush CAN’T make low polygon models. I think is not geared to that, so use every tool for its best using.
Here’s an example that it can, but at the end, some features are neded for this so specific stuff…
http://www.pixolator.com/zbc-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=003109 http://www.pixolator.com/zbc-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=000060
If you are making an isometric view with rol, or strategy (aoe, starcraft, diablo, etc) prerendered game(that is mid polygon count, as sprites are small, no need for extra high polygon count, but yes more than low.All is prerendered to BMPs), then it can help a lot in modifying the models you export from Max. You’ll reimport after the models modification (for example, adding tentacles, horns, making some magnet deformation, or characterizing someway) the obj with created texture also, again in Max. You will set up the bones (as you would in LW, Milk, or whatever) make the render in the 8 directions and that’s all.
My last advice test all the tools you can your own, be quick, have your mind really awaken, but be patient. You’ll find the one suits you for each tasks…You’ll end managing a LOT of them…At the end, you will be able to use them all, each time will cost less effort to learn.
Good luck.