I have tried it.

Justicar tank works OK and I have read on other sites about how they are capable of tanking raid mobs. My personal experience is that it seems to work OK but you have to outwear/outlevel an instance for a reliable smooth run every time. If you are lower level or same level, I tend to wipe a lot. Once you are 2-3 levels above, you'll do better.

However, I found that once you reach the 30's, I had a hard time with DPS and boss mechanics. Honestly, I am a below-average skill boxer, so you might have more success.