SSD prices are dropping so quickly that it makes less and less sense not to buy one for your PC. You can already find 120 or 128GB versions for £90, and for less than twice that you can buy an SSD with double the capacity. It will still be a long time before SSDs begin to rival traditional hard disks in terms of price per gigabyte, but the cost has now gone down enough that it's becoming more common to use them in higher-end PCs. With good reason too, because upgrading from hard disk to SSD will make your PC feel faster - more so than a faster processor or more RAM could ever do.
Most annoying slow-downs when you're using your computer are due to the hard disk. Programs that take too long to start up, a game level that doesn't load fast enough, and so on - these are all caused by your hard disk. Starting up Windows and other tasks can be done twice as fast with an SSD than a traditional hard disk. When several tasks are running simultaneously or when you swap operating systems the difference is even more noticeable.
SSDs have evolved quite rapidly over the past few years. Almost all current SSDs use the SATA 600 interface. That makes a big difference, since many SSDs can easily achieve read and write speeds above 300MB/s. It's safe to say that any SSD with the most current controller is a huge leap forward compared to conventional hard disks. There are also discrepancies between different SSDs, such as a variety of controllers, firmware and flash chips, and these all have an influence on read and write speeds. You can read the rest of 48 SATA 600 SSDs round-up on Hardware.info.