At 3 years I'm still very new to the industry, and as such have only ever used In-Design. Because like Dave has just said, it comes with the other Adobe products in the Creative Suite. It makes no sense to buy the CS then buy Quark on top.
That being said, I know some agencies that still use Quark, simply because they've been in the industry for so long it's what they know, and either don't have the time or have no need to learn a new program. Especially if they have deadlines to me.