Short answer: there's little reason to buy Professional as long as Ultimate is available; Ultimate is only $20 more and has every single feature offered by Windows 7, including some key features that you might regret not having.
Whereas Windows Vista had a criticized lineup, with lots of versions and confusing overlaps, Windows 7 has strict super-set SKUs (no venn diagrams) and essentially only comes in two retail versions: "Home Premium" ($200, upgrade $120) and Professional ($300, upgrade $200).
There are two somewhat more obscure versions: "Starter" which is a dumbed-down version which is only available when packaged with low-end PCs (notably netbooks) and then the souped-up "Enterprise".
"Ultimate" is a retail version of "Enterprise", the latter only available with volume licensing; there is no technical difference between the two. The exact differences between Pro and Enterprise are clearly described on Microsoft's enterprise page: http://www.microsoft.com/windows/enterprise/products/windows-7/features.aspx, in my paraphrasing (apologies for my humor, duly labeled):
- DirectAccess: VPN stuff for mobile users.
- BranchCache: File system stuff for hierarchical caching in big corporate networks.
- Federated Search: Corporate-wide searching.
- BitLocker and BitLocker To Go: New fun ways to encrypt your files so that when you need them in the future you will never be able to recover them (joke).
- AppLocker: Enhanced support to block application usage to increase CFO liability for unlicensed use of Microsoft Office (joke).
- Virtual desktop infrastructure (VDI) optimizations: better performance for Microsoft VM technology.
- Multilingual user interface: Allows one Windows installation to support multiple language interfaces in one installation.
In other words the Enterprise additions are, as expected, enterprise-oriented, e.g. security, remote features, vm stuff, etc. Nothing you would typically care about unless corporate MIS makes decisions for you. With one notable exception, the already mentioned MUIs (multilingual user interfaces). This feature was considered enterprise by MSFT because of virtual machines - e.g. enterprises want to be able to set up reference operating system images that can subsequently be reused in multiple language regions. It turns out that in our global world, many individual end users find this feature useful. But you might like the other features as well. I suspect some people might like the the BitLocker capabilities (in particular with respect to USB storage devices).
The retail pricing difference between Ultimate and Professional is minor ($20) so for an advanced user there's little point to even think about it. Personally, I'll pay the $20 to just avoid kicking myself later.