I have the new Fire 7". I bought it for $30, which is about all it's really worth to me. I find the interface to be convoluted and difficult to use and about as counter-intuitive as can be. And yet I use it. It's the tablet I keep always turned on, charging it every couple of days. It holds up pretty well and at that price I don't care if the battery dies in half the usual time.
I did install Google Play on it. As other said, it was easy. Download a file manager, then download and install 4 files in the correct order, reboot. That's it. Log in on Google Play and you're ready to go. Oh yeah, first enable 3rd party apps.
I had the previous Fire 7" also, purchased on sale for $33, and I did the same with it. I never had any problems after enabling 3rd party apps or installing Google Play. Everything worked just fine. Except the Fire interface, of course. That never worked fine.
If they come out with a new one in a year or two I'll get that too, assuming it's very cheap. It's not a very good device but everything works okay and it's handy having one I never turn off. It's always ready to use.
As for the 10" one, if the speaker is as improved as the screen I'll be interested. I watch a lot of Prime videos and my 8.9" Fire is getting old and has no micro SD slot. I'd like to replace that. But only if the speaker is very good. The speaker on my 7" Fire is pretty poor.
Barry