Improved posture is definitely a plus. I don't automatically slouch in my chair, but it's easy to slump without being aware. Standing isn't an automatic improvement. I sometimes lean in too much. But it's pretty easy to keep myself straight and my shoulders relaxed. And even though leaning in is not good, it's not bad to lean on the table top from time to time. And sometimes I find that I stand with more weight on one foot or the other, just as I would normally.
Even though standing eliminates the problems endemic to sitting, it's not absolutely comfortable. My hips feel better, but my legs and feet pick up the slack. Anyone who stands all day (such as many service workers) will tell you that it's not easy. One thing that's important for me to remember is to keep my knees soft—slightly bent. Locking them is pretty generally bad for any kind of standing. I stand on a chair mat, a soft rubbery one, but I'm also standing most of the day in my slippers. I might need to wear something more supportive on my feet.
And of course I need to take breaks. Which I do, as I always did. Now I take some sitting breaks. The only problem is that while my cat can no longer sleep on my lap, he's quite content to take over the empty chair. He's a big guy, and he's not too good at sharing a corner of the chair so I can sit. But we're working it out.
I think standing has improved my focus, generally. Somehow that part of my desk now feels more like a workstation. My personal laptop is no farther away, but it's on a different level. The top surface is all work for all of the work time. I do find, however, that if I'm short of sleep, standing does not keep me more awake in the afternoon than sitting. There's really no remedy for lack of sleep other than to get more sleep.
The Lack coffee table is about 17 inches high and not adjustable. I'm pretty comfortable with the height because I'm used to my desk being a little higher than some. I have never liked "keyboard drawers," which feel too low for me. Still, this setup might be a bit too high. If I have to adjust, I will do so. This table was cheap, and it's not sacred. But if I cut some off the legs (after measuring twice, of course), I will also add some amount of height adjustment. Further fine adjustment might be necessary.
I wear prescription reading glasses that are not quite bifocals but are called "office glasses" because the top part magnifies a bit less than the bottom. That's so I can read what's close to me and also see my screen, which is farther away. Now, however. my screen is even a bit farther away than it used to be. So far, my glasses still seem to be doing the job, but it's something I'm aware of.
One reason my laptop is so far away is that I use my old Dell keyboard plugged into it because the small Lenovo keyboard with no numeric keypad doesn't work as well for what I do as the full-size keyboard. The laptop screen would certainly be closer if I didn't use that keyboard. I'm finding that the coffee table doesn't absorb sound as well as the heavier table below does, so I might want to get some kind of thin pad to put under the keyboard.
As someone with a chronically messy desk, I must say that I'm quite enamoured of the extra shelf space. More room for piles of stuff! I've also taken advantage of space to bring the main power strip up off the floor. No more crawling under my desk at 5:30 in the morning to turn on the juice.
It's early days, but so far I'd say this experiment is a success. There are downsides, but I think they are outweighed by the upsides. I've never really cared about how sitting is supposed to shorten my life, but I do care about pain in my hips and butt. I think we're going better on that and staying just a bit more active in the process. There's nothing that says I can't dance at my standing desk!