There only a handful of FBs could solo VC when the FB top skill was Infernal Impact, simply because the class lacked the needed DPS. Now with the addition of new gears, new skills, pets, runes etc. That's but a distant memory and even low level FB can solo VC with ease. FB comp skill is one of the best skills in the game. It would be the best if FBs would have a better base attack. Assassinate is a fast skill, 3 seconds in normal and 2.6 in combo.
It has pierce effect and works great against high-def mobs even when you have crappy gear. It also has a travel effect, which means you can bounce through mobs to covera significant distance. It has orange text which means it has added damage. Overall one of the most, if not the most useful skills of the FB. You cannot build a PvE combo without it.
The fastest GM skill of the sword classes but still rather slow. Its DPS isn't terrific but it still has a nice range, a great debuff and DoT. Not the most powerful skills but still very useful. It also has a travel effect but it only works in a straight line. Also the limit is quite different. While comp hits almost 360 degrees, GM skill only hits forward, with the exception of very big mobs.
But that's where the GM skill can be more useful than comp. Because comp hits at 360, it doesn't have a very long reach, if you don't use the travel effect, in which case you might end up in a different place. The best example is at ic2 in fire traps, where GM skill can give you more exp than comp, despite the lower stats, because of the forward pierce damage effect, almost like a limit 3 lance. Just have fun in Cabal.