Visual Force was probably the biggest announcement at this years Salesforce user conference (aka Dreamforce).
To understand the benefits of VisualForce, it helps to first understand the limitations in Salesforce today. All users see the same screen, headers, sidebars, and navigation elements in Salesforce today. Data customizations occur within the boundaries of these UI constraints.
VisualForce does for web-based enterprise software what Power Builder and Visual Basic did for traditional client-server programming. It allows developers to borrow from a library of pre-built components, incorporate them into the layout of a page, and bind the user interface to back-end data methods and models (called Apex Controllers in this context).
In short, this is basically RAD (Rapid Application Development) for the web. VisualForce is currently an XHTML template language, but I suspect it won't be long before a true WYSIWYG drag-and-drop development environment is available that removes UI developers from the XML syntax altogether.
As a Salesforce Developer, we (3 of us from i-Dialogue attended) were just drooling at the opportunity to start using Visual Force, but as a Salesforce Partner, we're being forced to walk a fine line between partner and customer loyalty.
Apex, and presumably VisualForce, will only be available to Salesforce Unlimited edition customers. Our loyalty to Salesforce and this new development paradigm dictates that we should proceed forward, developing our content management solutions using Visual Force, and convince customers of the obvious value and benefits of this approach.
But loyalty to our customers dictate that we must also continue to be backwards compatible with both Pro and Enterprise Editions and enable rich customer experiences and online interactions using the features that are available to the lowest common denominator today.
My understanding is that VisualForce and Apex controllers will be available to all editions for an additional $25 per user/per month. I'm anxious to learn more about this.