Yep they are. Pretty much all the time.
There's ONE downside though, and it may or may not be important to you, depending on the game you're buying... and that's the refund policy. Buying off Steam directly gives you that 2h automatic refund policy no questions asked. Greenmangaming understandably doesn't have that. So if you're purchasing a game you're not sure you're going to want to keep if it turns out that it's just really bad, or tech issues or whatever... you might still want to purchase it on Steam itself. However, if it's a game that you KNOW you're going to love and