Do SSD hybrid drives perform better than HDD + ReadyBoost flash?

One major difference is that ReadyBoost is limited to USB 2.0 bandwidth (unless your computer has the ultra-rare and extremely bleeding edge USB 3.0), whereas the hard drive is on the much, much faster SATA interface.

Thus, putting fast flash memory on SATA alone is enough of a win to say definitively that it will be faster.

ReadyBoost is also designed around relatively slow I/O constraints, which limits the scope of what it can do, too.

The one review I found was quite positive. It does seem like, with the right algorithms, you could have the best of both worlds here -- the speed of a SSD (mostly) and the capacity and low price-per megabyte of a traditional HDD.

I reckon it will not be the technology that's a winner here, it will be the algorithm used to decide what to store where. Seeing as we don't know the algo for Vista, Win7, or the hybrid, I guess it will take empirical evidence to get a reasonable answer. Having said that, the OS can run more complicated algorithms, look at usage patterns over longer periods, and understand the filesystem itself better, so perhaps there's more potential there. One possible slowdown to ReadyBoost is that it has to encrypt everything because it's assuming removable media, whereas the hybrid solution has no such constraint.

"a hardware implementation ought to be faster than a software implementation"

I'm not sure that has to be true, but you get the advantage of knowing that if your computer's under a heavy workload, the hard drive will still operate at optimal speed. Edit: also it keeps your data buses emptier.

An advantage I see to ReadyBoost is that you've separated the two storage technologies, so you can update them independently as prices decrease or technology improves.