Like you Dazmond I had never used fan jets, that is until just over a week ago and I'm back to using pencils already.
The main reason is that I had to increase the flow rate for them to work properly so immediately started using more water.
yes you have yo put the flow up but the rising time is far quicker than pencil jets - one offsets the other
you would need to give it more than a week testing them out as you are used to taking longer with the pencil jets
the main reason foe fans is not water saving but better water coverage and therefore better rinsing over the glass
this gives better results and an added benefit it's quicker.
Downside is if you have stubborn deposit then you are using more water while scrubbing - however with a quick soak
at the start of the cleaning process on the window all but really terrible bird stike comes straight off.
real water saving is done through triggers or aqua daptors - and if using these then higher flow can be used as
your not using water anywhere but on the window
Darran