I will use the "user" intervals in the Gen 3 for these intervals. This accomplishes two things. At the start it will be easy to hold above your target wattage, the user intervals display your current and average wattage for that interval on the same screen. This makes sure that you don't start off too hot. At the end of the interval chances are you are going to be hurting a bit. Now seeing the average watts will do the exact opposite thing as the start, it'll be a goal and you'll have to push it not to let that number fall.
Here is an image of a recent hour long interval that I did (the date is wrong on this screen shot). As you can see at the start, the wattage held very smooth but then I got into some rolling terrain. It can be a very difficult thing to hold steady wattage when the terrain is going all over the place. Without a power meter chances are you'd be going too hard on the uphills and not hard enough on the downhills. Being able to monitor the watts keeps a good consistent effort throughout the interval.
Doing these intervals is a great way to improve your cycling ability. Immediately looking at the file you can tell right where the interval was performed (even if it wasn't highlighted in blue). This is the difference between training with power and riding around with a power meter on your bike.
Using this feature can also be very helpful in time trials and avoiding going out too hard and blowing up during the race.