I've had a similar experience. Although I still have some back issues, for the most part, my posture, etc. have gotten much, much better since I've started standing. My standing workstation is a total hack job--basically a small, cheapy (non-standing) desk with some Ikea parts bolted on to it. It's hideous but I work from home as a contract worker and am on a budget, so it works just fine for that. To sit, I've got a backless bar stool thing (it's kind of an odd stool). Sitting in it matches the height of my makeshift desk. It is also uncomfortable to sit in for great lengths of time, as I tend to slouch and my back gets quite tired without a seat back to relax into. I've found this to be a very good quality, though, as with a comfortable chair I found myself far too likely to forget to stop my sitting break and start standing again--I'd just end up sitting in a tall chair at a tall desk all day long.
Oh, also, you know those kinks you get in your neck sometimes when you sleep on your bed all wrong? Well, those have almost all but gone away since I've started standing. If I do get one, a day's worth of standing almost always clears it right up. I also feel substantially better and, while I generally need to take a 30 minute break after work to sit down on a couch and rest, after that I have plenty of energy. Of course, it's very difficult to measure happiness, but I will say that in general, the times that I've deviated from my standing work station have been promptly followed by less satisfaction both with work and in my home life. I simply can't imagine going back to sitting all day.