Well, here's the fruits of my researches:
SSDs work by using flash memory, which is actually very slow, but there is +a lot of it+ and it runs in parallel, so the overall performance is very high. This is important - any data is distributed across the drive, not in clumps like on a HDD.
Users typically notice that an SSD drive runs at the manufacturer's stated peak I/O performance at first, but soon after that it begins to drop. That's because, unlike a hard disk drive, any write operation to an SSD requires not one step, but two: an erase followed by the write.
Also, when an SSD is new, the NAND flash memory inside it has been pre-erased; Users start with a clean slate, so to speak. But, as data is written to the drive, data management algorithms in the controller begin to move that data around the flash memory in an operation known as wear-leveling. Even though wear-leveling is meant to prolong the life of the drive, it can eventually lead to performance issues.
That's still only a partial picture. More info here
http://www.anandtech.com/show/2738/8
It's quite a drastic reduction, but if you're using something like the OCZ Vertex (Intel is better but pricier) SSDs are already something like 15x the speed of an Apple hard drive, so even after a performance drop it'll still be more than 10x.
Of much more concern here are reports about Apple's own SSDs that appear to be made by Samsung. "On no, it's an OCZ." you say. Uh oh. From what I've read OCZ's Apex and Summit (the one Apple uses) use rebranded Samsung drives.
+"My new SSD died"+
http://blogs.computerworld.com/mynew_ssd_died_dare_i_tryanother It was an OCZ Apex.
+"What's Wrong with Samsung?"+
http://www.anandtech.com/show/2829/19 Those figures look terrible!
There's a lot of rubbish on the net about this from folk who should know the limitations of their own knowledge better. For instance there's a persistent myth about de-fragging, indicating a complete lack of understand of how an SSD works. De-fragging (I'm pretty sure) might actually +slow it down+ and reduces the drive's lifespan!
I hope someone from Apple can chime in here and reassure folk...