In the last few years the development of internal geared hubs has really taken off and flourished. There are a number of great developments which I will address in later posts.
I've put together a chart to illustrate how the gear range of internal geared hubs is different than that of bikes with derailleurs. (click on the chart for a larger view.) The number of gears isn't really of most importance. The range and spacing of these gears is.
The chart shows the gearing of a popular commuting bike called the Breezer Uptown 8, which has a Shimano Nexus 8-speed hub, with red squares in the blue band. The blue band represents the range and how it compares to other setups. I call this graphic a comparative ratio plot.
One common misconception with derailleur systems is that, for example a 27 speed bike ( Triple chainring in front and nine sprockets in back) has so many more gears and a wider range than say a 21 speed, or for that matter, an internal 8 speed. The number spacing between the chain rings and sprockets has so much more to do with it than the actual number of combinations.
You can see in the above chart the range of various popular derailleur systems compared to a nexus 8 speed hub with various chainring/sprocket combinations (ratios.) One thing to note is how many duplicate "gears" there are in many of the derailleur systems. Note the "traditional 10 speed" has only seven unique gear combinations and a similar range. I use as a general rule, the number of sprockets plus the number of chain rings to estimate the number of usable "gears." There are plenty of exceptions to this, but it is the simplest method I've come up with.
How much range do you need in your gearing? The great advantages of internal gears over derailleur systems is simplicity and reliability. That is a reasonable trade off for some high end speed or low end hill climbing gears you may not use. Internal gears can be geared as low or as high as anything else, but may not have the same overall range as more expensive derailleur systems.
