I posted some ramblings about my early experiences with the mirror class, Juggernaut, in this thread
. I struggled through to level 47 before finally parking him at the end of Voss, after I was stuck in the class quest there. In the meantime, I started a BH Merc who I have since got to 50 and finished the class quest with no problems.
My personal feelings on the class for PVE are that it is really only worth taking if you are either some sort of power player with a lot of MMO experience, or you plan to play him constantly grouped. Solo it just really isn't a lot of fun, unless you enjoy munching medpacks like candy and dying a lot.
Mechanics-wise, you don't start to get any useful tank abilities until around about the time you finish Chapter 1 of your class story, in the early 30s. By about level 40 you should have the majority of your abilities plus be fairly close to the top of your tree (Defense/Immortal). Even then, your two main problems will be a consistent inability to hold aggro and a distinct lack of survivability. These are obviously the two crucial parts of the tank role, which presents a problem. The latter can be mitigated by getting the best gear, running in a group with a healer, and learning how to properly use your skills. The former is just a broken part of the game at the moment which seems to be widely agreed on by most people playing the class.
If you are planning on playing solo, you probably have an advantage in some respects playing a JK over a SW in that your second companion who you get on Coruscant (!! Does any other class in the game get two companions that fast?) is a melee DPS. The SW doesn't get their mirror companion until into their 30s. As a melee tank with little to no ability to maintain aggro, I found that the a melee DPS companion was the best bet. A ranged companion will constantly be dragging mobs off you from 30m away, which is a serious problem. Melee companions will still drag mobs off you, but at least they'll usually be standing right next to you at the time, saving you from chasing the mob all over the place Benny Hill style and breaking your rotation. That sort of thing will kill you quick. For solo tactics, always have both you and your companion target a Strong mob first to kill it as quick as possible, then mop up any Weak/Normals. I have consistently found that two Strongs together (or worse, and Elite and a Strong) are nigh on impossible to tackle solo, unless you are over-levelled. Conversely, a single Elite is usually little problem once you have all your tools.