Do people actually buy into it - since if the returns were good why would they sell the software? Is there any public and verifiable evidence that it produces returns that outperform the market?
Yes, people use them. But do note that these trading bots are not something you install and let it run accumilate an income for you, it's far from that. It is hard to do day trading for sure. I've heard people using GunBot and Haasbot, main point with them is to apply custom strategies, so you will have to know something about trading to start with. Since market changes you will have to rethink and put different trading strategies, bot is usefull as you have pre-made code for you with nice UI to start with with lots of interesting functionality. But as I said you need strategies and this is why they sell those bots and prices they do.
If something is to good to be true, it probabbly is some kind of scam, check bitconnect for instace how they promissed extraordinary returns, while these guys like gunbot as you mentioned don't guarantee profit.