Uhmmm, essentially SSD's are flash memory. Pretty sure that has been tested quite well. How many of us have a USB drive?
Do you demand the same amount of hours per day of constant activity on a flash drive as you do a normal hard disk? Do you trust a USB flash drive as much as you do a hard disk?
Admittedly one thing that would cause bias in my answers to those questions is that I wouldn't trust a portable device as much as a non-portable, as there is a greater potential for physically nasty things to happen to portable devices because they get moved around.
I back up my files on a regular basis, but I still trust the hard disk to a certain extent. I would probably go for an SSD out of curiosity, price aside for a moment, but I can't trust that my customers fully understand the potential for a new technology to be unpredictable, and I don't want them screaming at me for recommending it in the first place.
Also, we all use pen drives, sure. But to how large a capacity? Also, we use them in special casing outside of a computer, what happens when they go inside a computer and are subject to different stresses? Might they react more disagreeably to say a power issue than a normal hard disk?
Changing the normal scenario for a device brings in new considerations. After a recent experience with a 2.5" HD plugged into a single USB port with no mains power, the hard disk got killed almost on the spot due to a faulty USB port causing the disk to power cycle multiple times per second, I now only go for either disks with two USB plugs or mains power, to offset this possibility. Hard disks are normally quite resilient to the sort of power issues that can happen inside a desktop.