In reply to Yanis Nayu:
If you do properly structured training and are willing to spend the money on one, then a power meter will almost certainly help you improve. There is a reason why all coaches prefer riders to have them,why all the pros use them and so many domestic road racers and TTers have them - they help you ride faster if used well.
If your riding/training is less structured, then you won't see as much of a direct benefit but you may want it purely out of curiosity, or to track improvement - again, it's just down to whether you want to spend the money really...who's to say that we can't have these toys?
Even if you aren't using it for structured training, over the months you will get used to what numbers you can put out, and will be able to use it pace yourself on climbs or into headwinds etc.
I bought mine a few years ago, then got into doing a bit more time trialling, where it made a big difference for me - both in racing and training. I do not regret buying it at all.
I'll put it this way - if you had £600 or so to spend on an upgrade to your bike that stands the best chance of improving your cycling then a power meter (+ a book on how to get the most out of using it) is money much better spent than, say, a pair of sexy deep section wheels.
I'll warn you though, using a power meter is a real reality check! I once saw what Contador's power zones were and realised that the power that I do on a 10 mile TT barely counts as a warm up for him; I would be there blowing out my arse, barely able to grunt, while he could ride alongside me chatting away merrily. Kind of lets us know just how good these guys really are!