For training, BGS is bad for damage/hr (exp/hr); it gets about 5-10k exp/hr less than a whip or SS. If you need a crush weapon, it's not terrible, but honestly a Z Spear or SS is better exp/hr as well.
For bosses, BGS is decent, but having a DFS or a soaking shield is very, very important. Besides, whenever you'd need to use a slash weapon, a whip is better.
The only useful Godswords are SGS (for monster camping) and AGS (for a spec in PKing). However, both of these are trumped by other things... monster camping is usually better with a Unicorn or with Soul Split juggling, and nothing can beat the spec of a Korsai's when PKing.