Some people either love or hate the flip screen. For me, it is not a deal breaker one way or the other; all depends on what you like.
I think most who are disappointed in the G15 is due to the sensor size and Canon didn't step it up to match or beat the competition. I don't think Canon saw Sony coming up and surpassing it with the RX100. Probably take them another year to have something comparable. Until then, the RX100 is riding the wave.
Without the flippy screen it's not worth it 'Techradar' seems to think so too with a non committal review.
If push came to shove I'd buy a G12 at the lower price but then it's not a lot different to my G11 which is working perfectly well since it's overhaul.
I was really hoping for something more radical The only major changes are a better lens and the loss of the screen which makes it a no go for me and I am not shelling out 500 quid just for the new lens, for my needs it's just not justified. I'd be replacing something OK with something mediocre!
Nikon just stole the Canon's thunder!