My William loves it too.
We don't work through it together much tho as I just don't seem to have the time with James as well and trying to do his school reading book and te normal painting, play doh and craft too
However - William does a lesson by himself and then uses up all the points he is awarded by playing the little games on there.
I would definitely set up the free trial! TBH - William already knew all the words that he is doing on the lessons that reading eggs have put him on and therefore not learnt much at all but it all re-enforces what he already knows (iykwim?) but I can definitely see how a younger child would progree through the stages and learn loads.
I thought it was a bit pricey - but it is only a one off really!
You could register them both if yu want - but you could also quite easily just register William and make them share their points (presuming you only have one pc and so can't play at the same time) as even though William was put on a higher level map - he can still access the lower maps and enjoys doing the simpler ones sometimes just for fun.
There was a write up about reading eggs on gurgle the other week.