No matter what you use Tor network for, whether you simply want to browse the web anonymously or find something from hidden sites like the Agora Marketplace, generally it is a pretty useful piece of software. It can be used whenever and wherever you need it. For example, you can easily keep a copy of the Tor Browser (a Firefox browser pre-configured to relay traffic through the Tor network) in a USB flash drive and use it when you are accessing public Wi-Fi.
But is Tor still secure as most people would believe it to be? There are recent reports that the FBI and other related agencies have initiated an exhaustive crackdown on most darknet marketplaces (and the closing down of Silk Road 2.0 last year was a very prominent example), including the arrest of 17 people. This is just a small number, compared to the millions of people who use Tor everyday.
The thought of the government finally infiltrating Tor is not really that farfetched, considering that the whole project was initially developed by the US Navy.
The answer though is a bit comforting for most Tor users. Tor is still very much reliable in this day age. It is important to note that Tor, from its government-backed beginnings, is a fully open source project. This means that vulnerabilities can be easily patched up as soon as people spot them.
So, yes. You can still safely use Tor and access darknet sites like Agora Marketplace if you want to. Just make sure that you anonymize yourself as much as possible (use a VPN with Tor Browser, not give any identifying details, etc.) and practice safe browsing habits.