Ive played the same SK since launch and i cant pull myself away from him.
Soloing, Grouping, Raiding, PvP,
Soloing id give SK's a 1/6 if you have the reaver ability and all your AA's maxed out in your reaver line ( minus the recast reducers if you dont want them ) as well as your Spell and Critical hit %
Grouping id give the SK 1/6 mostly biased opinion because I just cant trust another tank to do as good a job as I do. im sure every one feels that way about themselfs though. but honestly the self help heals with the reaver line combined with Divine aura and Deathmarch will save your party in some of the most extreme situations that no other fighter class i've seen being able to handle.
Raiding id give SK 3/6 as much as i hate to admit it seems like Guardians where made to Tank Raids.. However.. There is no reason why a Shadowknight cant do as good a job if not better then any other tanking class. raids arnt about how good your tank is IMO its about how well you work together.
and PvP id give the SK 1/6 against all other fighter classes. lots of your opponents try to kill you as fast as possible with stuns and stifles right off the bat... such as bruisers and monks but we can easily combat this with 1 minute 12 seconds of Immunity.. just long enough to destroy our opponants.. no need for harmtouch or divine aura 90% of the time. I LOVE to PvP... specially if there is a roleplay reason behind it. I have NEVER lost to a paladin, even paladins more fabled out then I am have not been able to defeat me.. and i only have 1 piece of relic armor a fabled shield and a fabled 2 hander. with the right tactics SK's can easily mowdown ANY Berzerker or Guardian. however.. watch out for other shadowknights... they are what might be your toughtest fights.
and thats my 2cents =)