I'm going to chime in and agree that for many and most people, Home Premium works and for those who need a few extras (like remote desktop or XP Mode), then Professional fits the bill. Unless you need things like AppLocker, BitLocker, Distributed Branch Cache, subsystem for Unix applications, or the Multilingual language pack...then you likely are paying more for stuff you won't use...as this is what Ultimate provides for you.
As a consumer, you cannot purchase enterprise. This is reserved for volume license customers only. For consumers, who feel they need the features provided by Enterprise, you can purchase Ultimate. This page provides a really nice overview and a good chart breakdown of those feature differences. Windows 7 editions - Wikipedia, the free encyclopedia
One tidbit worth noting, is that Ultimate has a shorter supported life cycle than Enterprise or Professional. I'm unsure why this is the case, since they are essentially the same OS. But Professional and Enterprise (which is what businesses would use) are supported by MS until 2020, while the Home Premium and Ultimate versions are only supported through 2015.
If you look at my system specs you will see that I am running Windows 7 Ultimate. And the ONLY reason for this is that Microsoft gave away free copies of Windows 7 Ultimate at the "New Efficiency" launch parties and that is what I am using. Had I purchased with my own money, I would have gone Windows 7 Professional without a doubt (As I use XP Mode and want remote desktop).