I'd say From Ashes is definitely worth it. The mission itself is kind of dull, but Javik himself is a worthwhile and interesting. Even if you don't particularly care about the lore, he's just a cool dude to have. They did a lot of interesting things with his character and played around with the idea of what it would be like to have the last survivor of an ancient race we know nothing about. Liara, who is kind of an audience surrogate in their first scene, went in expecting him to be the be-all and end-all, but what she got was an ordinary dude who also happened to be a bitter old racist. He's like an angry insectoid Dave Lister.
As for Omega...eh. I got it when it was on an XBL sale, but I wouldn't say it was that worth it. There was a cool gun and a nice, if simplistic couple of moral choices, but nothing much else.