I managed to snag an NES Classic the week before Christmas and I've been playing it since then. I don't want to review it or give impressions of it the way I normally would with a game or system I'm just diving into.
The NES Classic is just pure nostalgia in and adorable little form factor. It transports me back to being 5-10 years old again and I can't describe the joy I get from seeing my kids play it. They're just so happy playing the same games I was at their age. It's amazing.
If you have any nostalgia for the NES you should pick up a Classic as soon as they get their stock situation under control. You won't regret it.