My completion rating is close to 25% complete, but a good portion of that is side ops. I'm having a blast with the game so far despite the odd lack of story so far. However my biggest gripe is with Mother Base. I've built each platform up to level 2 and while it's a cool big base, it's so damn lifeless. There's nothing going on or nothing to do really. It's populated by a small handful of guys patrolling around having conversations once in awhile. Other than a few easter eggs, diamonds and the shower, I don't see any point to go there.

Maybe I just came in with the wrong expectations. When people were talking about how you'd be building up this big base, I pictured something along the lines of Mass Effects Normandy where you can have conversations with your crew members and see people working on different things. I don't understand why the studio would spend so much time making art assets for this massive base and then not fill it with anything. For anyone who's gotten further into the game, does the base become more interesting in any way?

There are a couple little events that occur if you go exploring in the right places, but largely what you see is what you get. It's still a huge step up from how it was handled in Peace Walker, where you could only get a sense of how large your base was from the model that updates in the menus between missions.
I'm not disappointed, but I didn't have huge hopes for it, either. I got what I expected.

I mean it's way more interesting than the Normandy ever was. It's a bigass base you can explore (and customize to an extent) at will and go give people morale boosts. Turns out being on an oil rig is not that glamorous even if Kojima thinks it is.

I think some things could have been done to make Mother Base more interesting, but I wouldn't go so far as to say I'm disappointed in it. It's cool seeing it grow and have more and more troops, vehicles, and weapon emplacements show up as I progress through the game. Given that there is so much to do elsewhere, it doesn't really bother me. Then again, I wasn't expecting anything from Mother Base either as I haven't played a Metal Gear game since the late 80's, and I knew next to nothing about Phantom Pain leading up to release.

The way the story gets tied to Mother Base is amazingly impactful, and that's all I'll say.

I enjoy seeing it grow and how my decisions subtly impact the overall look and feel of the base. It feels very sterile however. There's a few events that happen from time to time, but beyond that it's fairly empty. I also don't like the troop morale system (I really don't want to walk around a base punching people for 10 minutes). I much prefer the Mess Hall mechanic.

while it's true there's not a lot to do in the base itself, I think it sorta sets the stage for the FOB missions that come later in the game.

Well, after all it's based on those menu in Peace Walker...