I've tried Bing on several occasions.
It's a good search engine, but as with most users,
they aren't unhappy with their results and therefore will find no need to switch. There really isn't anything "broke" with their competitors' search methods- yet. Sure, search is evolving rapidly and results are bound to change and perhaps Bing will get smarter. But for now, I'll stick with Google.
I'll stay with Google for not only my satisfaction of their products (Gmail, Google Search, Picasa, etc.) but primarily, for me- I enjoy that they give Microsoft a run for their money in a number of areas. Microsoft, pre-Google was a slow, lethargic giant that HEAVILY resisted change to benefit the consumer.
Firefox pushed Microsoft to finally get serious about browsers again (seriously, would you want to have countless years of IE6 limbo?), just as Google is pushing Microsoft to strengthen their online efforts (notably Office Online and Windows Live). Without Google putting pressure on Microsoft, we wouldn't have Office Online, Bing or half of Microsoft's latest attempts to regain power.
Google may be a "monopoly", ONLY because of a GREAT product (call it the "cool" factor or whatever you want Google haters, but seriously, the product rocks by it's own merits.) Google doesn't lock in advertisers nor do they block you from searching anywhere else. But, if Microsoft (a proven monopolist that has ABUSED their power) got control of the internet, it would seriously be game over again for years without any serious competition. No other competitor has the power to make Microsoft paranoid about their own legacy products as well as the online offensive. For those reasons, I'll stick with Google.