ok, ill answer your questions...
from what i claculated this circuit is good in using for ac voltages over 20v, this is when the effficency is better than using a single resistor.
you might not want to put the resistor, R1, it is for current limiting, but for a green led, it works fine without it, but since for ir leds, and small voltage leds, it is a little bit hard to find a zenner diode for the right voltage for led.
and, finally, the value of C1 depends on what voltage you use the circuit for(sory i firgot to mention it), but make sure that you need the right voltage capacitor, depending the ac voltage you use the circuit in.
i have something made like this, but with a highter voltage zenner diode, and 4 12mm yellow leds.
and another thing, use a 1W at least power for the zenner. it is not that much power disipated, but i found out than 0.5W burn, quite fast, i think it is because i used quit old ones, but i am not sure.