One of the intel Itanium designers showed up in my neck of the woods working on a Ph.D, and he naturally gave some talks. He made a very convincing case that at a given clock rate, the Itanium was about 3x better performance than conventional RISC designs. That left a few pesky practical issues on the table, like:
Q: when shipped, will the clock rates be the same? A: no, Itanium was clocked 3x slower, negating the architectural advantage.
Q: can you buy it? A: no, Itanium was complicated and very late to market.
Q: is it cost-effective? A: no, Itanium was 10x the price of convention chips and ran Very Very Hot
This week, the high-performance crowd is chasing GPU offload, since CPU clock rates have stalled on amd64-style CPU's.