@pentaxuser, keep in mind the drawbacks of thiourea already mentioned (i.e. it's a powerful fogging agent as well), plus the ones not yet touched upon, in particular its somewhat suspect safety record. Compared to what you'd otherwise find in a typical rapid fixer (i.e. amm. thiosulfate and sulfite), it's not particularly benign (although it's not exactly a horror compound, either). I can imagine this, in combination with the by all means adequate effectiveness and speed of regular rapid fixers, precluded a widespread adoption of a 'better' alternative.
Indeed, my own interest in such a 'superfix' is more academic than practical; I don't really see how scraping a bit off the few minutes I usually spend fixing film will make my life all that much easier.