I was much more a diesel enthusiast until I saw the mpg achievements of gas powered cars of late.
I test drove a 2009 Jetta TDI over a year ago and thought it was the bee's knees. I thought it was very refined and amazed by 60mpg (50mpg us). The price was a bit of a turn off and living in a place where the temperature can drop below -30 degrees for several weeks a year I was somewhat leery about the cold start potential.
Flash forward to when I'm actually ready to purchase a car in 2011. I learn the Hyundai Elantra is rated for 58mpg! (48mpg us). Running on gas, way cheaper to purchase and without the worry about the cold start. Other cars like the 2012 focus were not far behind. It seemed then that only a great fool would buy that diesel Jetta which doesn't really seem to bear an advantage whatsoever anymore. What slight fuel economy advantage the jetta may have you could never make back on the purchase price.
I don't know if the diesels are encumbered by emissions standards or if gas engine technology just advanced at lot faster as of late. In any case I'll be as happy as a pig in...mud, when I finally receive my 2012 5-speed SEL
To the original question, would I buy a diesel Focus? Only if it clearly showed a cost savings and traditional die hard diesel durability.