I bought FSD in 2018 when it was only a $3k upgrade over the $5k price for basic stuff like lane following. I originally ordered our X with FSD, just in case it went up in price a lot, but i recently just took it off my order. The normal FSD is not bad for highways, but FSD beta seems pretty terrible (my experience so far is only a few minutes with a friend's car). I'm guessing it is still multiple years out before being really useful. It just isn't worth $10k right now, and i think will barely be worth that when it actually functions correctly. I do wish I could transfer it from my 3 to the X though, since it will probably become our road trip car... I think the only thing i'll really miss is the ability to lane change without taking it out of lane following mode.
I use FSD daily. I also purchased it when it was low priced at $2K. The only commute I currently do is to drop off the kids at school which is a 10-mile commute each way through city streets (no highway). I will say before describing my experience is that you have to get used to it. You can't just ride with a friend and expect to understand it. It's hard to explain what I mean by that, but you just learn to know your car and its behavior. Yes, it's far from perfect, but it's improving.
Now, about my experience. If you were going on a straight road for miles on end, it would behave just like it does on highways... flawlessly, including auto-lane changes. You do have phantom braking from time to time and I even have radar on my M3. However, I think the latest updates have disabled it (I'm on the latest update). It recognizes lights well and has run a few 'late yellow/early red' lights, but I made sure there were no cars around while letting it do its thing. It also recognizes the speed limits very well and adjusts without hiccups like it used to in the past.
So, where does it suck? a) construction sites. it just doesn't know what to do half of the time and errors out, b) turns (left/right). it acts as if it is scared to make the turn and even asks you to press the pedal when you feel it's safe at times. If it's a turn on flashing yellow, good luck with that. You will be sitting waiting forever from my experience. It always wants a green light. And c) in neighborhoods, on roads where it's an L road, but has a rounded corner at the turn, it freaks out for me. Thinks it's a road going into a house. Weird stuff. But keep in mind that my list used to be much longer. It has improved with every update. It hasn't tried to kill me in a few weeks so that's good. By that I mean when I first got it, it tried to go against traffic a few times.
Is it worth purchasing with your MX? That depends on your situation. If you think you'll be keeping this vehicle long term, then sure. Why not? If you plan on upgrading once the new batteries come out, then no. You can just subscribe and pay a fraction of the cost, hopefully. I personally did not add it to my MX because I personally don't think it's worth 10K at the moment. If I ever need it for a road trip, I can easily subscribe for the month. And if Tesla ever announces a new price hike, they are usually good at giving a heads up and you can purchase it then.