It was just okay. From a story standpoint, I feel like it was a diversion that Shepard wouldn't have taken on. I mean, they could barely take back the station, how many resources are Aria really going to be able to provide the Alliance for the attack on the reapers? It was a lot of fun spending time with Aria and Nyreen is the best new character since Mordin in ME2. It's a shame she went out like that and an even bigger shame that we didn't get her in the main-game.
As for the new enemies, the new Rampart Mechs were pretty cool. They were tough until I equipped incindiary ammo, that tore right through them. The Adjutants, on the other hand, were just kind of lame. Especially when you learn their origin. I would hit them with dark channel and follow up with flare from Aria and they were toast. Speaking of Flare, it's wicked awesome that we can get that or lash as a bonus power!!
All in all, I don't regret getting it, but $15 was too much. I enjoyed Leviathan infinitely more. I'd rate it somewhere around Arrival and Kasumi. It's no LotSB, that's for sure. For the hardcore ME fans, just get it and make your own opinion. For casual fans, wait for a sale.