I think the King Crab is worth is, yes. It's probably supplanted the Atlas as my favorite mech though I only have about sixty total drops in it (if the stats are right - and I'm suspecting they're not since I'm sure I have more than 2 drops in the 000B).
I probably run about the standard as can be expected.
KGC-000(L) (KDR 2.23 in 20 matches (seems like more) - 29-13):
2 x AC/20 with 7 tons
1 x LPL
1 x SRM6
1 x SRM4 with 1.5 tons
STD300 with 1 external DHS
*Quirks are just too good to consider using a different ballistic, but opinions may vary. I use range and cooldown modules and they really help with pumping AC/20 rounds into people.
KGC-0000 (KDR 2.3 in 38 matches - 46-20):
2 x Gauss with 4.5 tons
4 x ML
1 x LRM15 with 1.5 tons
3 x SSRM with 1 ton
XL300 with 2 external DHS
This is probably a somewhat odd build, but I've found it to be pretty effective. Gauss obviously works. The four medium lasers obviously work. I use the LRM15 as a suppressor here and there, whereas the SSRMs are good at shoeing the lights away. The missile cooldown quirk helps with blasting out streaks. I use Gauss cooldown and advanced zoom and I'm lucky in that I'm pretty accurate with the Gauss against lights from a distance. This is in my drop deck for CW and many of my deaths no doubt are because of that - I've not found much success in CW yet as I drop alone. However, I did get seven kills in CW in this mech in one match. Then again, perhaps CW doesn't count towards stats. I don't know.
KGC-000B (KDR 1 in 2 matches - 1-1):
3 x ERLL
1 x LRM20+A
2 x LRM10+A with 7 tons
STD300 with 4 external DHS
I'm not sure what to do with this guy, but I need more drops in this with this loadout to get a feel for it. I think the lasers can be effective; not sure. I haven't played around with it enough yet.
All in all, I love the King Crab. It's slow, but it doesn't feel slow. It's effective in the brawl because it's nimble-ish. High weapons mounts have been stated, but it's worth stating again that these are highly advantageous. It's smaller than some mediums, height-wise, which is sort of weird when you're piloting a 100 ton mech. I get good damage with the four medium lasers without risking my body, and I feel the ERLL will be good to go once I get an idea as to how I want to drive that build. Is it XL friendly? I don't know. Probably more so than many assaults. I know I've died from losing a ST in the 0000, but I usually dish out enough pain before I pop. I'm averaging 297 damage per kill in the 0000 so I think I'm putting down accurate, effective fire though I'm sure some of that damage is a spread as a result of the LRMs and streaks. The mech is able to do a lot of different things quite well, depending on the role you wish to take. LRMs, sniping, hill-humping, brawling, or a mixed loadout. I'm already converted, but I'd recommend picking one up and seeing how you like it.