There appears to be a lot of misunderstanding about what this is, what it does, what it doesn't do and how it works. I'm possibly confused too, however this is what I understand things to be.
1) Intel Rapid Start does NOT boot your system.
2) It is a replacement for Hibernate/Sleep.
3) S1 S2 S3(sleep) S3H S4(Hibernate) & Intel Rapid start are all variations on the sleep function.
4) Boot, Booting up etc is described as from when your computer first powers on and goes through the hardware system checks, and initialises your hardware.
5) Resuming from a sleep state is NOT booting
6) No one can boot into a used windows7 OS in 5 seconds.
How does it work ?
As before with Hibernate it writes the existing memory to disk, and powers down your system. When power is re-applied it loads the image, bring you back to the same place in windows, documents open etc..
Whats special/different about Intel Rapid start to existing Hibernate/sleep?
It does not require power like S3 Hybrid Hibernation.
It appears work in practically the same manner as S4 Hibernate.
It appears to be more efficient, and faster than S4.
You will need a 3rd party utility from your motherboard manufacture, possibly a bios update and new drivers for windows.
You will need to have a special hidden partition set for this to work.
As with S4 sleep - no power is needed.
What is all this S3, S4 etc. nonsense ?
These are sleep states.
I'll probably get some part of this wrong.
S3 - Currently mostly referred to sleep mode in windows. This is the fastest sleep mode.
The computer powers down the CPU, however leaves the ram powered. When it wakes, the system jumps back to life quickly.
Positives - FAST and reliable
Negatives - Great for short term sleeping however it does consume power, Desktops usually still consume a significant amount of power in Sleep mode.
If power is lost so is your work (unless you saved). A full reboot is required.
S3 Hybrid (Most common)
As above the data is maintained in RAM & written to the Hibernation file on disk.
This allows for the speed of a S3 ram wakeup, and the redundancy of S4 (hibernate) to
This is a combination of S3 and S4 where data is saved to ram and a dedicated hibernate file on your HD.
Pro's
Generally reliable
Reasonably fast
Con's
Still requires Power
Requires HDD space
S4 - Commonly referred to Hibernate
This mode Dumps the contents of Ram onto a dedicated file on your HDD.
Pro's
Power can be removed, so your work is safe.
Very low power consumption is used in this state (our electronics always use some even when turned off)
Data is safe
Con's
It's Slower to resume than S3 states
People with older HDD's maybe limited in space and disable the hibernate option, as this takes up same same amount of GB as your RAM.
To conclude:
It makes no difference to boot speeds.
Your hardware must support it
Is it better than existing sleep modes? - Yes.
Will it save me money? - In the long term a small amount if you are replacing a S3 sleep mode.
Whats all the fuss about? - Clever marketing, It's just another sleep mode similar (yet better) than what is currently available. Faster resumes than existing hibernate.
It is also a "Green" technology, the less power we all consume, the better.
Have I tested it? - No. My motherboard's bios supports it, however no application has been released yet to configure it. I will probably update to it when available.
Gigabit NIC went into 100mbs power saving whilst asleep, I sent a magic packet and it woke up 
