A couple points worth clarifying. Flash is a type of memory used in all sorts of storage devices. For example, Solid State Drives, USB/flash/thumb drives, and so on. Flash is available in different technologies which affects services life, speed, reliability, and so on. USB flash drives are generally much slower. Typically people buy them based on how much storage you can get for the price. In any event the performance is limited by the I/O technology (USB 2 and 3). Science experiment: create a USB 2 flash memory boot drive and see how much slower it is than an HDD.
SSDs, on the other hand, have many factors that determine their speed and reliability, including type of flash used, controller firmware, I/O channels on the drive, processing cores, and so on. As noted earlier, anandtech has reported on these for years, and can provide much detail, as can tomshardware. Other than price, it's hard to find anything wrong with SSDs -- they really improve the overall experience.