I have a google voice number before, but I didn't find it useful.
Actually, you can register a virtual phone number, and then use VPN to apply google voice number .
It's totally free.
Not necessarily. Google bans VPN's after a few uses. Sometimes if you are lucky you won't get caught, but it's better to have an organic number from the USA.
Unlimited free texting? Are you sure?
I am thinking they must be free to a certain limit after which you'd be required to pay some fee to send texts?
Can these be used to make calls or receive calls/sms??
Yes, this is unlimited free texting, (to the USA and Canada) no limits. They can also be used to receive calls and sms, making them useful for phone verification.
I like the concept, but one thing to make it better is reload all those google voice numbers with some balance, so they can be even used to make calls with them
They don't need to be loaded with balance, as these numbers give unlimited free calls and texts to the USA and Canada. Balance is only needed if you want to text/call countries other than USA and Canada.