Yes, read Hunter and Coggan's book. You can also start with a quick summary here: http://home.trainingpeaks.com/power411.aspx
But in terms of your questions
do you still get cadence, speed and heart rate readings ?
All the commercially available power meters display and record cadence and speed and support HR monitoring and recording if you wear an appropriate HR strap.
why is power so accurate to different trainings scenarios ?
Power is what propels the bike, not HR, not even cadence depending on gear choices and not pedal force again depending on gear choices and cadence. Power is what you want to improve and there's an old adage that applies to many things that basically says "what gets measured gets improved'. You can measure HR to assess workout quality and pacing in some situations (not very good for short intervals or bursty high power work) but you can't really 'improve' HR, your HR at LT or FTP or VO2 Max pacing is basically what it is beyond some early adaptations and long term aging processes. But at that same HR good training coupled with sufficient recovery can lead to dramatic increases in power and again that's what drives the bike. So measure the thing you're most interested in improving not secondary or tertiary things that aren't directly related.
In scenario terms:
- Peak power or very short duration power in sprinting situations and how you best generate that power (e.g. high cadence, lower gearing vs. high gear, lower cadences) is very useful information that can help riders learn to jump quicker and sustain higher speed sprints
- In short interval situations power is a very accurate reflection of the work performed. HR basically doesn't respond quickly enough for things like one or two minute anaerobic efforts.
- HR has inherent averaging and long term lag and drift characteristics that can lead to suboptimal interval or TT pacing. It takes five to seven minutes for HR to reach the average during a 20 minute iso-power interval, if you try to hit the target HR zone too quickly you'll have to overcook the start which usually leads to fading before the finish. Similarly HR will drift upwards during long iso-power efforts. If you try to artificially cap HR to a tight pre-defined training zone you'll have to back off power in the final minutes to offset HR drift, again that's suboptimal pacing for training intervals or races.
- Energy in kj burned is directly related to power and can be accurately measured with a power meter. With a very small variation due to an individual rider's Gross Metabolic Efficiency that energy burned in kj is a very accurate estimate of dietary calories burned during exercise and far more accurate than on line lookup tables or HR based calorie estimators.
There's more but that's a pretty good start.
which one is more convenient: hub, chainring or pedals (the new one by polar and look) ?
It depends on your situation and finances. Crank based PMs are great in terms of allowing the use of any wheelset and if it's a modern crank design (like Shimano Hollowtech or SRAM) then swapping cranks between bikes pre-equipped with suitable bottom brackets only takes a minute or two. Hub based designs are probably the easiest if you have multiple bikes but not so great if you run different wheelsets for different conditions like deep wheels or a disc on race days. I wouldn't be an early adopter of pedal based systems and would give those technologies time to work out any kinks.
I ran Ergomo of about six months when I started using power and it was a nightmare of drifting values and no way to objectively check accuracy. I went to PT hubs for a couple of years and was very happy but as I accumulated more and more PT wheelsets I decided the economics weren't working well for me. For the past four seasons I've been running wired SRM units that I've picked up on the used market as a lot of folks upgraded to wireless. That has been very cost effective and I like the flexibility and the ability to use any wheels. If I had to buy these new I'd probably still be using PT hubs as they're great but all in all I'm glad I switched to SRM as it's been very trouble free and easy to field calibrate with known weights so I'm very confident about the accuracy.
There's no magic in a PM, it's just an instrument to measure the work you've performed. That said, the PM can give you some very objective feedback on your training and racing and really pinpoint what is or is not working for you. You've still got to do the work and sometimes what the PM tells you isn't much fun to see like when your power for a certain type of interval or racing situation is lower than you'd hoped but that is also information that can tell you a lot about recovery and freshness that may help you manage your overall plan or remind you to dig deeper on your focused training days.
-Dave