I'll be taking delivery of my X in a couple weeks and have only test driven both, never owned. My reasons:
Space. While others have noted you can have more length in the S with the seats folded, I find the X provides better functionality in offering both storage and maximizing seating simultaneously. If going on a ski trip, in the 6-seater configuration, I can have the skis and boards down the middle with a slight angle and still comfortably fit 5 (with just one rear seat down). Also, I have a big hockey goalie bag that's just a few inches too big for the S trunk, so again, I could fit my cargo and still fit a family if needed. Plus, you can seat more than 5 adults if you get the higher seat configurations.
Falcon-Wing Doors. By all accounts, they do offer more issues, but there is nothing else like it and for that reason, I like the uniqueness and am ready for some servicing if needed. Love it or hate it, it's all about knowing what you're getting into, and I like how they stand out.
Features. The X just has more smaller features to it such as the automatic doors. Again, this kind of touches on my previous point where it's just such a unique car. Elon himself said how it was a mistake to load the X with as many additional features as he did and won't be making that mistake anytime soon. If any other automaker didn't know that already, I bet they learned from his lesson. With that said, it's safe to say there won't be any car quite like it for a long time.
Windshield. At first it didn't seem like such a big deal to me, like more of a bonus, but then I was fortunate enough to take an overnight loaner X after placing my order. Even just over that time, I became so accustomed to the extra visibility that once I went back to my BMW or any other car, it just wasn't the same and felt cramped. Not sure how else to explain it, but it just feels more freeing.
Of course there are cons, such as slightly reduced range, a higher price tag, and an increased likelihood of issues to fix, but if I'm getting a car with such a price tag, I need it to have the functionality I require, which only the X meets. So while you can always come across the horror story of a bad delivery, I found that you can typically get it resolved fairly easily nowadays. Again, just know what you're getting into regardless of the decision you make.
Hope that helps!