This post sums it up nicely.
I drove an 2009 Sonata SE with options up to the teeth tonight, and a Elantra 2009 GLS.
I love the power, smoothness, quietness, leather, refined interior cabin, and V6 of the Sonata. The Elantra is nowhere as refined as the Sonata, and it had quite a bit more engine & road noise, however the handling is a bit better than the Sonata. What set the Elantra apart from the Sonata is that I like the Elantra exterior better, and the dash lights, gauges, and buttons are absolutely beautiful. (VW, Toyota, and Honda should take a page out of Hyundai book).
I have an appointment to test drive a 2008 Altima, 2009 Mazda 3, and 2008 Civic Si in the coming up weekend, but at this point the Hyundais are far a head in the lead.