This project is an electric bike conversion of my Gary Fisher commuter using the eZee geared hub kit.  This kit includes a geared front hub motor, a motor controller, sensor electronics, throttle, battery and battery rack.  The 2007 cost of this conversion is approximately $1250USD total.  I've listed the pros and cons of this configuration below and provided some photos of the conversion.

Pros

1. Front hub installation is easy compared to installing a rear-wheel drive (because of the gears).
2. The torque characteristics allow installation on regular non-reinforced front forks.
3. It is modular, and any of the components can easily be swapped out, making it easy to upgrade/try different designs.
4. You get complete instructions and wiring diagrams, making it possible to hack the controller.
   
5. Using a brushless, geared motor is a good price/performance/reliability balance.
6. The setup has good range and speed:  with a 36VDC Li-Ion battery, I get about 20miles at 20MPH (assisted).

Cons

1. Front drive may slip, as opposed to having your weight over the driving motor.
2. Geared motor's whine might annoy you, whereas a non-geared motor is pretty much silent.
3. If you upgrade to a more powerful motor or a bigger battery, you must reinforce the front forks.
4. Needs a cruise-control throttle so your hand doesn't get tired holding the throttle.
5. The battery is the trickiest component to purchase.  Consider initial price, weight, capacity (Ah), voltage (~bike's top speed), and lifetime in recharge cycles.
   

Voice of Experience

1. Don't expect the 36V version to be a motorcycle.  It doesn't require much assist, but you get much better performance if you pedal.
2. For me, a major advantage is fast acceleration from a dead stop, which makes it much easier to drive in heavy traffic.
   
3. Use heavy-duty tires (like Armadillo) and keep them pumped up to the max every day to get optimal performance.
4. Conventional thinking makes low weight a major factor in biking.  In electric biking, weight is less important than structural integrity because of high speeds.  Ironically, for more powerful motors, a cheap department store steel-frame bike is best. 
   
5. Getting the best performance takes practice.  You have to balance using the motor with assist pedalling.
6. Be sure to plan your commute so you don't "run out of gas".  You can pedal without battery power, but it's not much fun.
7. Watch out when taking off uphill and in wet weather.  The front wheel tends to slip.
8. My first "upgrade" suggestion is to go from 36V to 48V.  The 36V controller can handle the voltage delta and you get about 5MPH more top speed.  You may need to reinforce the front forks.
   

Hub Motor Detail
If you have disc brakes, you need a disc conversion, and you need to adjust the brake calipers.

Battery Gauge

Throttle
You might need to bend things to get the throttle to work, especially if you have click shifters.

Motor Controller

Controller Hookup/Battery Carrier

Battery
The battery is the heaviest and most expensive component... about $500USD and about 10 lbs.