Lots of riders are into watts these days. Measuring watts is probably the best way to gauge a riders current fitness. Most of the riders measuring their wattage are doing it on indoor trainers like the Computrainer,Tacx, or the Cateye.
The most important thing to consider on wattage measurement is repeatable accuracy. There are two basic types of wattage measurement - electronic trainers, and on-the-bike devices like the Powertap and SRM. The trainers are the most INACCURATE!!! Even Computrainers have to be factory recalibrated, and they can cost over $1000 U.S. Now think about the Tacx and the Cateye costing in the $300+ dollar(U.S.)range.
Trainers can also suffer huge calibration errors by the abount of tire contact (pressure)the roller makes. Too much or too little pressure can seriously effect the accuracy of the wattage reading. When using a trainer, ALWAYS use the same wheel with the same tire, adjust the roller pressure the same every time, and keep tire pressure constant every time. Doing so will ensure your wattage readings will at least be consistant as long as your resistance device is working properly.
I've heard of some name-brand trainers being more than 60-80+ watts off actual watts. That's a lot of watts!!! Your 400 watts might only be 320!!! I've seen lots of posts by amateur riders saying they are cranking out 400+ watts at AT (anaerobic threshold).
When Lance Armstrong became World Champion in 1993, he put out 340 watts at his AT. It's doughtful any of these amateurs would have been able to finish the World's wheelsucking the whole way, let alone win the race!!!
I'm not bagging on electronic trainers. Indoor training is one of the best ways to get speed on the bike. Having accurate wattage data is valuable. Use it as a barometer, but not an absolute. Finally, what do your watts mean in the real world? 1500 watts sounds great, but on a flat road with calm wind, can you spin a 53x13 at 95 rpm for 10 minutes??? What's your top speed? Can you hold 60+ km/hr for 200 meters? That's real, and not subject to calibration errors.