The big thing about this from my perspective is that it abolishes roaming. One plan, and you pay the same whether you use it in your home country or anywhere else. I'm not aware that anybody else is offering that, especially not at that price.
As a German living in the UK i currently juggle two SIM cards, and whenever I fly back home I have to mess with the data flatrate there since it's worth activating the monthly flatrate plan even if I just use it for a day. And then I have to make sure that I don't forget to cancel the flatrate when I leave again. Not to mention that this means I have two phone numbers, and I'm not reachable on at least one of them all the time.
Roaming will hopefully soon be abolished throughout Europe (http://www.bbc.co.uk/news/world-europe-26866966). Although there have been talks that this might be delayed by a couple of years.
Then in the UK there's Three who basically abolished roaming in 18 countries (sadly not Germany for now). This includes unlimited (25gb) 4g, which is pretty awesome and better than the 256k Google is offering.
T-Mobile already offers basically the same international plan. The only catch (and it is a headache) is they cap the international data at 128kbps, whereas Google is capping it at twice that.
Regarding juggling sim cards: I recently traveled between a few EU countries using a 3 UK pay-as-you-go card (with the their roaming package http://www.three.co.uk/Discover/Phones/Feel_At_Home). It worked well, and even in the middle of nowhere in Italy, I had good coverage. It was a PITA to top-up before I left from the US. But if you are already in the UK, that is probably a non-issue. I think it was 15EUR for unlimited data + N/minutes & texts.
I am sure there are probably downsides, but it might be something to look into while Fi is still in infancy.
As a German living in the UK i currently juggle two SIM cards, and whenever I fly back home I have to mess with the data flatrate there since it's worth activating the monthly flatrate plan even if I just use it for a day. And then I have to make sure that I don't forget to cancel the flatrate when I leave again. Not to mention that this means I have two phone numbers, and I'm not reachable on at least one of them all the time.