Since this series is about a decade old now I'm not going to bother with the story introductions and whatnot.
Starting with Extra. Firstly I want to say I'm not your regular hyper critical and ocd anime enthusiast. In general I don't really care how overplayed a theme is, how cliched the characters may be, or how 2deep4u the story may or may not be. I can enjoy something thoroughly like EVA but equally enjoy something silly like mitsudomoe.
With Extra, it a pretty ridiculous over the top harem style story which honestly made barely any sense. But I really enjoyed it. There was a strong comedic pull throughout the opening of the VN that hooks you on the characters. It allows you appreciate the characters more easily for when they introduce the drama-romance parts of the story. But its really the comedy that I kept going back for and it didn't disappoint. The story was a bit wonky, but the script, voice acting, and sound effects more than made up for that. Towards the end, the individual routes become more prominent and to be honest I didn't really care for most of them so I skipped them. But these really only take fruit towards the latter half so you don't miss much if you skip them, or don't even bother.
Originally I was actually going to jump straight into Alternative but decided to take the translators advice and play through Extra and Unlimited first, and that was definitely for the best. Well, required is a better word.
I think the transitioning from that stupid, funny, and ridiculous world of Extra into the super serious world of Alternative is a pretty good literary device. I don't know if it was originally planned by the writers, but it really plays on your emotions. the shock of seeing all those characters thrusted into such a dire story. The main character (ie, you) has continuous flashbacks to his "old" peaceful world, devoid of war and tragedy, which only heightens your sense of emotion for all the characters. Unlimited served as a good introduction to the world of Alternative. I say this because of all the crazy shit that happens in Alternative. Unlimited seemed so pacified by comparison. But when I finally got to Alternative, it was an endless rollercoaster of ups and downs. Going through and seeing all the horrors these characters experience, you do become alot like the main character, you just want to go back the light hearted nature of the original Extra story.
The universe was exceptionally crafted, perhaps too much. Amazing thought and detail went into designing the world and the story's events benefited greatly from this. Perhaps a bit too much for some people, but I enjoyed. Mission briefings for example, could well go on for 20, 30 minutes, explaining in excruciating detail as if you were actually there, and even though they would never actually show all of the mission in the following scenes. Part of me wanted to keep going purely to appreciate the detail the writers created for this original world.
I haven't played many VNs but there is no denying MuvLuv is unique among them. I am not at all surprised it is one of the highest rated VNs ever. Personally speaking, being mecha/action/war orientated is a direct draw for me; I could never really get into something more 'normal' like clannad for example.
And now I'm sitting here wondering why in the world they decided to make an anime out of Total Eclipse and not these originals ! :|