Is it fair to say that up mixing stereo CDs is unlikely to be worth the trouble if the source is studio bound pop music but more likely useful if using well recorded classical (most of my music is 70s pop which is rarely available in multi channel format).
The situation is that it's a world of almost unlimited variation of options. Today there are 3 main players in the upmixing game Dolby Surround, DTS-Neural, and Auro, all three can do a multichannel 2D mix or a 3D mix using overhead speakers. Step back just a couple years and Dolby/DTS offerings sounded quite a bit different. If your looking to do a basic ambience enhancement the Logic 7 codec does some of the best I've heard.
Then depending on the recording, how each will perform compared to another will vary tremendously.
There just isn't any hard answer to your original question.
Personally my music interests lies completely in the Blues,Rock,Country genre. I now use some type of upmixing on just about everything I play, can't remember the last time I switched all processing off. But then again, not only does it vary with the recording I'm playing but also with the mood I'm in, at times a minor ambience enhancement, others something more dramatic. The best news is depending on your system front end, all the options are available with the push of a remote button.
I like it, YMMV
I usually prefer straight-up Dolby Surround 5.1. For me DTS Neural seems to pull too much stuff to the middle channel.
They both pull pretty hard to the center, but Auro does so in a dramatically less manner, and it also has a couple of switchable options in that area.
The home theater people talk about bringing up the center channel so that dialogue is audible. I would be interested in trying to improve "imaging" if there is any possibility of doing that.
Hard one. Place to start is just with a lot of experimentation with the stereo speaker positioning and some room treatments to limit side reflections, etc.
You also might look into DSP room correction. It can have a powerful effect on imaging.