What version of Premier are you considering?  I ask because some of the extra features are specific to certain editions.  For instance, the Contractors edition has enhanced change orders and tracking of subcontractor insurance expiration dates.  Because of the difference between versions, we usually recommend that our clients purchase the Accountants edition, which allows you to toggle between all the editions.

We work with a lot of job costing clients (contractors, builders, architects, engineers,  nonprofits that have grants, etc.) and though they love the additional reports that come with the Contractors, Professional Services and Nonprofit editions it's the little things that make them switch. 

The biggest, and most surprising, reason – you can turn estimates and sales orders into purchase orders with a click of a button.  It's hidden under the Create Invoice button, but once they discover it they're in heaven.  No more double-entry, no more data entry errors.  Speaking of sales orders, that's another reason why they switch – you don't get them in Pro (note:  you can also turn Estimates into Sales Orders).  They also really like the Invoice for Time & Expenses feature.  Yes, you can do the same thing on individual invoices by clicking Add Time/Costs, but they often forget to check all the tabs and if you're using job costing correctly there will likely be billable expenses on the Items tab.

You can also assign price levels by item to customize prices for different groups of customers or jobs.  If you have inventory, you can create inventory assemblies, bill of materials and units of measure.  For this reason, all business that have all but the most basic inventory should be using QuickBooks Premier or Enterprise.

If you work with an accountant, and you should, they'll love the adjusting journal entries and the reversing entries.  Premier also keeps a record of all your previous bank reconciliations which helps them find discrepancies.

Premier also has forecasting and business planning tools, including 5-year financial projections and What-If Analysis.  It also comes with 1 year of remote access instead of 6 months for Pro.

Best of all, the price difference on Amazon isn't all that dramatic.  Pro usually settles in at around $100 and Premier at $200.  It's a little higher now because 2010 was just released, but prices should stabilize soon.