I've used the ondrej/php to upgrade a VVV box (the standard WordPress dev environment) - it works very well.

If anyone's using Debian (assuming you're on Jessie – the stable release) you want DotDeb – https://www.dotdeb.org/instructions/
On Debian, PHP 5.6 (from Jessie) and 7 (from DotDeb) co-exist nicely without any warnings, you just install the php70 packages on top.

I didn't even bother to check if that was possible and went for an all or nothing approach; test all my sites in PHP7 on dev first, push the code on production, make all the nginx configs consistent (including moving the PHP section into a single shared file), then finally update that to point to the new php-fpm socket after php7 was installed.

What I could (and probably should) have done, is install php7 first, leave 5.6 running, then switch one site over at a time - easy with Nginx because it doesn't know anything about PHP, it just sends the requests wherever you tell it.

I've not noticed a reduction in CPU load, but mine was very low to begin with. I'm yet to upgrade a production which is running WordPress, but it's super fast on the dev VM.

Incidentally, surprisingly few people have watched this talk by Rasmus - loads of useful tips in it.