SD card reliability

How reliable are 128gb SD cards? I am thinking of using one in a SONY
vaio ultrabook with 128 Gb ssd drive as extra memory but at $100 or so I
would hope it to last.

 

All flash memory has a finite number of write cycles, but unlike SSD drives
where they've developed newer and better technologies to greatly (by two
orders of magnitude) increase their longevity, common SD cards are still
made with older flash technology.

For storage use, you'll have no problems - but if you're going to be writing
to it frequently (definitely _don't_ designate it for storage of your
swapfile), you'll find it failing sooner rather than later.