It isn't as quick as a scraper, but it does get it off, if you notice it, just wet it and clean a couple of other windows before going back to it. You can also tip the brush onto its edge and use the plastic part of the brush to give it a bit of a scrape if there is a particularly stubborn bit of poo
Also, it isn't as if this is something you are going to find on every window either.
I've been going close to 12 months now with WFP, and I honestly can't recall having had a problem with bird poo, even on high windows, and yes, you can see it from the ground
Ian