You should remember that they sometimes give direct translation in the show, which means complete shit ass like in episode 4 of Hidan no Aria IIRC, each and every word was word to word translated in the same order making it even more hilarious, although GG did make a few good releases, as mentioned above, Durarara! and Madoka were pretty good, but that's 2 shows out of past 4 years of their releases, which is pretty pathetic.
Good fansubs are :
Ayako
Doki (90% of the time)
UTW (nearly always)
Mazui (nearly always)
Chihiro (cuz I like Kristen

and Chihiro are good in overall)
EveTaku (sometimes)
Ahodomo (new with very good translations)
and more
the bad are actually the ones that seriously don't give a damn, like GG
oh and HorribleSubs are no fansubbing group, they only rip from CR and Funi. their releases are usually good, but the subs quality completely depends on CR/Funi quality.
oh I have to mention as BAD group Eclipse, because they ceased activity, that is the only reason why they are bad XD