No speed improvements as of yet, mostly because almost all SATA hd's (except for the Seagates and those new Maxtors I've been talking about in the above post) are just IDE hd's with a parallel to serial bridge chip. So they can't be faster than their IDE brothers, it's exactly the same drive.
The only benefit right now is the thin cable. However, if you would decide to buy one of those new NCQ supported drives, AND you've got a motherboard chipset that supports it as well (i915 or i925x), it's a whole different story alltogether.

As for relieability. I would say they equally (un)relieable as IDE drives. They should be, as said above, they are the same drives. (for now still anyway)