Graham over at The Inspired Library School Student has posted a good review of EndNote versus Zotero. I like Graham's approach: he took a typical user task, writing a sample essay and using both programs to build identical bibliographies. Among his conclusions:
The two programs are similar enough that anyone already comfortable using EndNote should not switch, unless they are paying to use EndNote, in which case they should certainly consider Zotero as a free alternative.
Personally, I disagree -- though I've been using and teaching EndNote for several years, I found Zotero so much easier to use that I switched. Its ability to grab citations from non-database/OPAC sources like the New York Times and Amazon, and its consistent saving interface across multiple sources, makes it worth the switch to me. And of course you can always grab citations using Zotero and export to EndNote if you must.