Is the latest addition to the Mass Effect universe necessary? Honestly, it really depends...
(The following does not contain any spoilers for Leviathan, or Mass Effect 3)
Chronologically speaking it is very hard to pinpoint when Leviathan occurs. Since I was on a subsequent play through I began the mission before the Tuchanka ark, which may not make a lot of sense, mostly because Banshees appear much later in the game. But as far as developments go it doesn't really matter. If you have already finished a play through it won't matter anyway, you already know what you're going to encounter later in the game. The DLC itself is simple enough to access. You recieve an email from Admiral Hackett, and after that you head for the Citadel. You go to a new area. As far as I can tell you only have the duration of the DLC to explore it, so make sure you explore as much as you can whenever you're there. The DLC will take you to 3 new worlds. All of which are visually beautiful, especially the last world. You will only be facing one enemy faction, the Reapers, and combat is about what you would expect. I was using my level 60 Vanguard, on the Hardcore difficulty, and I didn't have much trouble. Unless you're on a first play through it's business as usual. You will face all the Reaper enemies, even a Harvester so be prepared.
In terms of extra goodies, you will get 2 new weapons, a new assault rifle and shotgun. Most of the extras you will be picking up are new weapon mods. There is a new one for the assault rifle, and a few for SMG's and Pistols.
From a narrative stand point, Leviathan is strong. But it does feel sort of stand alone. The ambiguity of the DLC's placement is also a curse. We know a lot of the endgame revelations, but Shepard (at least in the part of the story) does not. So when the big revelations are revealed, you know you will be getting a gimped explanation of some of the big reveals. I don't know how this will influence the endgame, but from what I heard it doesn't really add much. In terms of Mass Effect 3, it feels isolated. In terms of the Mass Effect universe it's huge. The things that are learned actually had me reevaluating my choices made at the end of the game. It's so much to consider, but it doesn't factor in anywhere else. I feel like Admiral Hackett should mention the enormous findings every time I talk to him. I feel like the crew should be losing their minds over this enormous revelation, but once the mission ends, things like Cerberus and the Reapers become the main threat again.
And that's another problem, it feels like Bioware still has secrets about the Mass Effect universe. To me, it was fine when the narrative ended. But adding in extra DLC to raise bigger questions, only to have them remain mysteries seems a little dumb. It's hard to know what it is a set up for, especially with the sense of finality we receive when we complete the game. Say what you will about the endings, but the game makes it clear that it is finished. Leviathan just comes in and opens up something else. It's still fascinating, and delivered wonderfully, but you just can't help but wonder what the point is.
As a DLC mission, Leviathan does justify its price. It's 10 dollars. I finished it in about 2 and a half hours. And I felt satisfied with the length. Most people seem to be completing it within the 2 and 3 hour window. The mission isn't all one go like the other missions in the game. You will return to the Normandy and the Citadel about 3 times, so you have time to change equipment, and weapon mods between sections. I'm pretty sure you could do another mission in between the DLC missions before actually finishing the DLC, if you wanted.
The DLC's soundtrack is great, fitting well into the game's already stellar soundtrack. The presentation is also a strong point. Just like Lair of the Shadow Broker in Mass Effect 2, this DLC will take you to some great looking locations. The last location on a stormy ship is particularly breathtaking. One strong point is that this DLC is fully voiced. Squadmates talk, and remark about the mission, meaning you will get unique dialog depending on who you bring. Even Cortez is a chatter box in this DLC. Performances are strong as usual.
Is this Biowares best DLC for the Mass Effect series? In my opinion it is not. I still give that honor to Lair of the Shadow Broker, Mass Effect 2's biggest DLC. This episode feels more close to Project Overlord in terms of structure, and narrative progression. I was disappointed that Leviathan made no effort with bosses. I would have loved a boss fight similar to Tela Vasir or the Shadow Broker.There was one point when I thought we would be fighting a slightly more powerful variation of the Reapers, but I was wrong. For the most part you're just going through the motions. The games setpiece moment has Shepard entering a diving suit to explore a mysterious ocean, but even that is short, which is a shame. The exploration in the new Citadel area is fun though. But the standalone feel the DLC provides, next to the mostly familiar combat scenarios makes for a strong yet familiar experience.
So, is Leviathan a definite purchase? Well if you are still madly in love with the series, and you play it regularly, the extra additions to story and narrative are great. It's good supplemental material for fans. Is it necessary information? It's hard to say. If you're on the fence, I would say maybe. If the idea of buying DLC seems unattractive, watching a Youtube play through of the DLC will also give you everything you need to know. But if you want to play through a new mission, you can't go wrong with Leviathan.