Kevin, 

I'm guessing you are talking about 6.x helpdesk ? we never implemented that systems end user page(much much like moving at the the early 90's) 
We bought Workflow about a year ago and did a quick incident request form that submits to the 6.x helpdesk. This will allow us to have pretty good submit page until we move to Service Desk 7.0.. The best way to enforce is when people email the helpdesk they get a reply with a link to submit the request by the link attached

We use it in Helpdesk 6.x.  I don't see why users shouldn't be able to submit incidents.  I've answered your questions below, but let me know if you're still wondering something about Helpdesk!

How do you get the end-user to use the Helpdesk user portal?
We use the Helpdesk user portal as an additional method, but by no means the only method.  You need to tackle it as more than submitting incidents online -- it's also viewing incident history, checking incident status, viewing the KnowledgeBase.  It's about letting users putting in a request for help immediately, without having to wait for IT to process an e-mail or answer a phone call--or worse, retrieve a voicemail message.  (And when they e-mail technicians directly, don't they lose track often?)

We drive traffic to our Helpdesk user portal all Altiris Helpdesk incident correspondence("Click here to view your request!"), in our e-mails to the user community regarding issues ("For updates, please check <URL>"),  in our call ACD message, in our business hours and after-hours voicemail messages, and through Internet Explorer shortcuts on managed Windows systems.  Approximately 10% of our incidents are submitted through the web without ever having made a push to customers to use the user portal through a special e-mail ("Hey, don't call us, use this portal") or telephone push ("Did you know I could avoid talking to you if you'd just go to this website?").

What fields are available for the end-user to complete?
Our Winuser portal isn't customized much, if anything -- we're doing ServiceDesk 7.x soon, and will do the majority of our customizations there.  We did brand the Winuser portal.  Everything can be customized, and a lot of it has been done before and posted about online by other Altiris administrators.

The default fields for us are name and e-mail address (auto-filled based on AD logon), Phone (we don't store this in AD), Title (a textbox for the user to describe the issue), Category (they select the best guess), Priority (how they rank their issue), Asset (a drop-down where they select their computer from ones Altiris has detected that they use), Attachments (where they can upload files like screenshots or documents), and Comment (where they describe their issue in more detail).

What are the amounts of follow-up calls to the user for more information?
We've favored a simple form which allows users to submit the initial request.  The downside is that we don't request very specific permission (e.g. "Which ERP database forms do you need access to?"), which means follow-up calls to the user.  But we've never minded this, and it's not a disadvantage compared to other methods that don't involve IT (e-mail and voicemail require a call back) or methods that do (phone calls are already requiring IT time on the phone).

What issues arise when you use the Helpdesk user portal?
I haven't seen any issues specific to the user portal.  I don't see any real downsides.

What issues are prevented by using the Helpdesk user portal?
The Helpdesk user portal is expandable.  It can take hundreds of requests a day, something we couldn't do by phone or e-mail.

One thing to keep in mind is if you have people in your organization without email. It's not a show-stopper if they don't have email but those users won't receive any email notifications that Helpdesk has built into it by default. They can still use the portal to enter tickets, view ticket updates, etc.

We have heavily customized our Helpdesk 6.x environment and alot of the work we do in the Helpdesk is based on emails back and forth to the end-users. We have custom-built "Ready To Close", "Pending Information" and "Pending Approval" processes, etc, that heavily rely on email. In fact, I currently have 123 different Notify Rules in effect to give you an idea of how much we depend on the email communication in Helpdesk. That's why we have decided for any users without email in our organization (call center reps) to deny access to the portal. They are supposed to have their managers (who all have email) to enter any issues for them in the portal.

Just one thing to consider in a large organization