TOR is as secure as you want it to be.
It isolates all traffic from your ISP. HOWEVER. It is not always endpoint secure (where the endpoint node exits to the general internet) because anyone can run an endpoint. Its well known that the US government themselves runs a lot of TOR endpoint nodes to vacuum up traffic. And of course any accounts/identifiers you use will still be traceable.
It is certainly much better than clearnet. But it's not perfect, and it's also slow as balls (never seen an exit relay run faster than 200kb/s)