I've got one, it's a great little tool, far more powerful than it looks!
Got up a ladder a few times to clear out the odd swan neck, but nothing drastic.
Spinaclean are also excellent to deal with.
I'm like you, busy with the windows but sometimes the custys ask, so I am happy to do it.
It's all extra dosh!
Also, on Saturdays, Mrs Window Cleaner likes to go food shopping (and I don't!). Nuff said!