I'm a big fan of low dose mescaline, but I also tend not to complain about "half-way turned on" states with psychedelics in general. No, I don't think low dose mescaline is a replacement for MDMA, rather it is a superior alternative. I say this partly because MDA/MDMA actually tend to fizzle for me, producing a rather unpleasant and anti-social melancholy state. I also don't like that the best effects are short-lived, and the long-term after effects (after at least 3 days, sometimes more like 7-10) are not so good.
Meanwhile I find low dose mescaline to be markedly stimulating, "heart-opening", and also peaceful and sublime.
I don't think it's as pro-social as MDMA is for most people. The only times I really enjoyed MDMA was when I had constant social stimulation, but with mescaline, I can have a tremendously positive experience in solitude. On a couple of low dose experiences, I went "adventuring", basically riding the bus to a museum or other point of interest. At the low level, the experience felt very controlled.
Keep in mind that it takes at least 4 hours for the experience to fully develop. Uneasy feelings may develop during the first few hours, and it's good to be familiar with that aspect and/or to stay in a safe location until you really settle into a positive experience. Moreover, even low doses can stimulate and keep you awake for up to 12 hours. It's usually best to dose mescaline within a few hours of waking up.
As for dose, I think 75-200 mg is roughly the range you want to aim for. Much beyond 200 mg and the experience becomes more properly psychedelic, and you get a whole new layer of effects with each additional 100 mg or so. At around 400 mg or so I think is where the experience begins to assume profound qualities.
Ironically my first two experiences with mescaline (in cacti) were low dose, and I couldn't help but compare them to MDMA in terms of the stimulation, euphoria, emotional opening, and so on. The irony was discovered in my third experience which probably came in close to 475 mg. At that level, it's just a completely different thing, like 100X as strong as the lower dose experience but also fully psychedelic with all kinds of visuals, "mescaline songs", and so on. To the point made by
@xdrc about cost, the issue with low dose mescaline isn't so much that it's too expensive (in so far as I don't use it that frequently) but that the experience is so very much more at doses that use only 2-3X as much material. I'm also personally blessed with access to a variety of other phenethylamines which produce different kinds of entactogenic and psychedelic effects, pretty much all of which I find superior to MDMA as well.