I had the recent opportunity to troubleshoot lack of performance on a Windows Vista laptop. That laptop had always been very slow... With 1GB of ram and two CPU cores, it is swapping to disk. There were periods of heavy swapping which resulted in non-responsive applications.
I disabled/removed many memory consumers such as McAfee anti-virus, Windows Media Center startup tasks, and some factory-installed HP applications. I removed Shockwave, as websites have been phasing out use of that bug-prone software.
After installing the modern Firefox browser and virus cleaning, I made a restore point and went on to making a backup with a portable external USB drive. Upon plugging it in, Windows Vista asked if it should be used for ReadyBoost. This got me thinking about the potential of using the mostly-unused media memory slots for a permanent ReadyBoost drive.
ReadyBoost works in conjunction with SuperFetch, which watches OS usage and preloads frequently used files. If those files are large and sequentially read, a traditional hard drive is likely faster. If those files are small or use non-sequential read access, a flash drive may be useful. On RAM memory-constrained machines, SuperFetch can place those files on ReadyBoost, possibly increasing performance.
In summary, ReadyBoost may be useful on machines with slow internal hard drives (Windows Experience Index less 3 or less are candidates), and constrained RAM, and no desire to increase RAM quantity (maybe the RAM slots are full, or laptop RAM is not worth the price).
Note that Windows Experience Index seems to be missing from Microsoft Windows 8.1.
Note that Windows Vista can only use 4GB of space. The size limitation has been increased up to 256GB, with eight separate 32GB drives in Windows 7.