I am going to try and make this quick, because I really want to make it back to playing Fallout 4 as soon as possible. I reached 20 hours, I estimate that I'm about halfway through the main quest, but it's hard to tell, because it's been at least 10 hours since I last pursued it. I'm just having too much fun poking around ruins and doing random sidequests.
This is ordinarily where I'd make a post trying to summarize my thoughts about the game, but I think it might be too soon to do that here. I still have half the map completely unexplored, and of the half I have explored, I've got 3-4 uncleared locations for every one I've thoroughly emptied. I don't expect to spend less than 50 hours on this game. Luckily, that should only take about a week.
My plan going forward is to keep doing one post a day. I'm starting to feel like my initial character build was mistake. I focused heavily on Charisma, for the extra speech options, but haven't really gotten enough out of it to justify shorting Strength and Endurance. But now that I'm approaching level 20, I'm starting to feel like the game's level scaling is outpacing my character. Fights have been getting steadily harder, and I'm chronically short on ammo (usually, by this point in a Fallout game, I'd have reached the tipping point where ammo is abundant).
I think what I'm going to do is try and finish the main storyline, and then write one, big spoiler post dumping all of my thoughts about it. Then, I'll start over with another, more exploration-focused character, and try to be systematic in my approach to sidequests.
To sum up: At 20 hours, this game gets a big thumbs up from me, but I feel like I've just barely begun to play it.