I don't think Extra is bad, like so many people say. I just think it's nothing special, and would probably be pretty forgettable if it was standalone. On top of that, it's in the same series as Alternative, which IS something special and extremely memorable. It's hard not to compare the two for most people.
That being said, I'm one of those people who only played the 2 main heroines so I could move on... and I can't remember, but I think I skipped most of Meiya's (and Meiya was hands down my favorite heroine by the end of the Muv-Luv series). The romance stories found in the actual routes just aren't my style, but I did enjoy the common route where it wasn't quite so serious, and focused on the humorous side. I'm not bashing them or anything, it's just that I personally do not enjoy them.
I enjoyed Extra for what it did for the overall story, by building up the characters and creating an attachment for the player. If I hadn't played Extra, I probably wouldn't have cared at all for Takeru's struggle (not going into detail to avoid spoilers).
Also, Miki is available as a route in Unlimited. She's not available in Alternative, for which there is a very good reason for. I'll leave that for you to figure out the reasons for, since you asked for no spoilers.