My MacBook Air has a small SSD and there is no way to upgrade it, I've recently started carrying an SDD, in this handy $18 SSD optimized external storage case.
I use portable USB HDDs for all sorts of storage: music, video and back-ups. They aren't very reliable and die on me far too frequently. When a disk recently expired I decided to try an SSD. I had a spare Crucial M500 240GB SSD waiting to be used and the Inateck case claimed to be optimized for SSD, so I gave it a try.
Installation/assembly was simple. You slide the SSD into the case. Close the case. Insert the cable into your computer and format if necessary. Under heavy loads, copying 1GB or larger files, I get around 28Megabytes/sec throughput, sustained. It is quick enough. Small file reads and writes feel instant and like I'm using an SSD, not a USB drive.
As there are no moving parts in the SSD I hope/suspect this'll last a lot longer in my backpack and briefcase. USB disks typically have 1-2 year lifespans for me, tho a few are years old. I am enthusiastic this is a slightly more expensive and reliable upgrade on portable storage.