I don't know about Mormonism. Reading calcsam's articles, I get the impression that the superficial archaeological gloss provides some intellectual respectability. But more generally, I get the impression that right now the Mormon community is still young and functional - like the early Christians, who really did provide a lot of charity, form loving accepting communities, pool their resources, etc. (And lost it as they grew. Any successful startup can sympathize.) If this is so, then we can expect to see their growth level off at some point. Early Christianity began losing it by the 300s or so, which gives Mormonism plenty of time left (but on the other hand, they grew much faster).

How memetically fit their beliefs are now, consistency-wise or appeal-wise, I don't know.

With Scientology, they have an interesting esoteric hierarchy of knowledge, which has long been a drawn to humans (think Eliezer's Conspiracy universe, or the Christian Gnostics, for that matter), and a number of half-baked Western & New Age derived techniques that apparently do work - a religious Toastmasters or pickup artist movement, you might say. (I think Luke posted an article on this. Could probably find it googling the 'Scientology stare'.) They haven't been that successful that their success stands in need of explaining; if they are still around in a century and have more than 10 million members, say, then they will be much more interesting a phenomena.