I came back to ME after a few months of not touching any of the 3 games just to experience the new Levi DLC and the Extended cut.
To be honest you can´t compare LotSB to Leviathan. Why? Lair was without doubt the best DLC in all of the 3 games not only story wise but also design and mission wise. I still remember the mission with the sky cab chase, that was pure epicness. Leviathan was kinda fun and I absolutely loved the new lore you gain about the Reapers and how everything started out with them (my good old friend Harbringer)
The DLC lacks a few more missions or exploring tho. I like the concept of those search filters but it would have been nice if you would need to go to a few worlds, look for Leviathan you know just to stretch it out a bit. Overall it felt a bit short (Only has 3 real missions the rest is in the lab). Worth the money tho
One point why you can´t compare Lair to Levi is also that Levi lacks a
true "endboss" fight. The Shadow Broker fight was extremely good and entertaining to be honest. The Leviathan story however does end too fast and abrupt. You know you dive down, meet Levi, ask him some questions and everything is done and Levi does not play any role after that *dreams of Leviathan fighting against Harbringer in the final battle on Earth - that would be an epic showdown*
The new DLC is in my top 4 of all ME DLCs (Nr.1 Lair, Nr.2 Arrival, Nr.3 Overlord, Nr. 4 Leviathan) however it is not quite on par with Lair, Arrival and Overlord both story wise and mission (fun) wise.