You should complete the DLC as soon as you can after Priority:Mars. 

You get a squad member Javik. But he will only be available after Priority:Mars.

IMO he is a very valuable squad member with very unique dialog options. I was able to do it fairly early in the game. His [Dark Channel][1] works wonders for me.

Adding Javik to your squad does net you some unique dialogue options when interacting with characters. (Not posting it here. But checkout [Javik/Unique Dialog](http://masseffect.wikia.com/wiki/Javik/Unique_dialogue) here.)

If Javik and Liara are on the mission Priority:Thessia, then that conversation adds a lot of lore, since at that point you are at a Prothean place where they posted a record of their attempt to fight the Reapers, as the Asari call it a temple and Javik is a Prothean. 

One last note, you can not take him on Priority:Palaven.


  [1]: http://masseffect.wikia.com/wiki/Dark_Channel