obviously you can still kill someone with a GD
GDs are still able to provide damage. In a wide open area, I don't think anybody moving at mounted speed has much excuse for dying to the GD alone.
Everybody I killed had more than 1 pass at me, and required more than an "all kill" to finish off. The equalizer is the fact that I can't move at mounted speeds with it out, so finishing anybody off requires them to expect the GD as the only source of damage.
In PvM, especially solo PvM, GDs are the best choice to solo the new stuff with. They're just too 'tanky' to ignore.
In PvP, they leave you too vulnerable (if you're not a garg/ninja) for me to want to specifically PvP with them.