I've just got the update prompt on my Win7 system as well. I prefer to wait until they've confirmed that they're not planning on foisting a subscription system on us
Also, what's the plan, is Win10 going to go straight from a "technical preview" to release?
Just got the notification on both of my 8.1 gaming PC and laptop,as to technical preview well there are quite a few builds of Win10 out as you probably know and Microsoft will decide on RTM build.
There might be a few more builds available then Microsoft will decide which Win10 build to optimize for RTM IMHO.
As to subscription that is not what they are doing,if you don't install the free upgrade to Win10(for 7 and 8 home users) within the first year then you will have to buy Win10 if you want to install or upgrade your OS to 10.
I think they are just trying to get people to upgrade from their old 7/8 OS ,making it free if you upgrade within the first year is good way of doing that.
I've no issues with 8.1(very solid for me) but looking forward to Win10.
Side note: as to the 29th July release ,if that is true(ie no delays etc) then Microsoft probably have a very good idea on which Win10 build they'll probably go with.