Not a single player game
No, not because it's a game where you have to play with other people but because i think this is where gamespot missed the point. If i were to play the game by myself, which i did for quite some time, then i would 100% agree with Gamespot.
This is due to the fact that Gamespot did not delve into AvA which is the heart and core of Global Agenda: to fight for territory, to play competitively and to ultimately just have fun with friends. Agencies are not only places where you do AvA but also where you can find people to help you level your low level character (thus pretty much getting rid of the 'grind') and actually find teammates that you can rely on.
Furthermore, you don't have to be level 30 as gamespot claims to participate in it as you can join one of the zerg teams where all you have to do is be there. To play on the frontlines, yes, being level 30 will help but to join in AvA? having some low level teams in your agency is also part of it.
The last point i want to make is that HiRez (the makers) do not just sit there raking in the cash with this game: they are actively changing the game and listening to player suggestions and complaints.
Some of the issues that were in Gamespot's review have already been resolved (such as the queuing) and there have even been inside jokes from the forums taken into the game itself.
Overall, if you are going to play by yourself and do your own thing, then i would say that ga is not worth the money that it's priced at but the bigger picture of what ga is trying to do makes it great