Luke W.
Google
Its not that big of a place but we were pleasantly surprised at how much they actually had going on in there. There's quite a few exhibits and as others have said, the models are pretty cool.
It's free to enter, and there's no way to actually make a donation or anything so you don't feel obliged to give, and don't feel as though anyone is expecting you to give.
There were two staff on when we went and they were friendly on welcome and exit and didn't bother us other than that.
Its not life changing but I would recommend going if you're in the area.