When I built the child haulin' bike below I used a three speed internal hub because of a couple of circumstances. 1) It was affordable, 2) came with a coaster brake, and 3) (related to #2) I didn't have the time to repair my MB-1 frame to use a cantilever or v-brake (one of the rear canti-posts needed to be reattached after a car collision). Since I had the chance to choose my own gearing I put some thought into it. Stock three speed IG setups seem me to have a decently low first gear, but have a third gear that is too high, and a second gear that seems too low. The "correct" cruising gear seems to hover between second and third, but you can't get to it. Correct cruising gear may be a matter of opinion, but I submit it isn't too far away from the low to mid 70 gear inches (5.6ish gain ratio if you use that method) that works for fixed gear or road singlespeed bikes. That gearing will get you moving at 20mph with 90rpm pedal cadence. In my book that's plenty fast, so I worked on the assumption that the I'd set my highest gear at this level. Using Sheldon's Invaluable Internal Gear Calculator, I found that with my tire size I could achieve this gearing with a 46x22 setup.
One coincidence of this experiment is that I'm now smitten with the Nexus 3-speed hub. It works very well, and I prefer it to its SRAM counterpart--easier setup, easier shifter action, better shifting. If that weren't enough, Nexus cogs are readily available in 16-22 tooth. This was important for me because I was installing a IG hub on a bike with nearly vertical dropouts. The wide range of cogs available allowed me to calculate (again with some internet assistance) the chainring/cog combo that would give me the gearing I wanted and get the needed chain tension without horizontal dropouts.
Back to gearing. I'm now adamant that this is the way to gear a three speed hub--especially on a city bike that doesn't exactly excel in the out of the saddle pedaling department. You don't need a larger gear, if you're going downhill you simply coast. Second gear is used for starting up on flat terrain, first gear is for going uphill or starting on an incline. Second is also used for cruising on slight inclines into strong headwinds. Third is your ideal cruising gear. 20mph is plenty fast for most riding. You can spin faster if you'd like, but you rarely need to. The lower first and second gear were essential for this bike, because with Silas in the Bobike Mini getting out of the saddle is very dodgy. But even without the child seat (I use take the seat off and use the same bike for all my errands around town) the gearing works very well. Low gear is 40 inches for the curious.
If you're thinking about building up a bike with an internal hub, try it out.
Commercial content--Nexus Inter-3 hubs with shifter are $80 (36H only). Complete wheels built by me start at $162 and include the shifter. 19T cogs are standard and included in the price. Nexus cogs 16-22T are available for $10.