Everyone should have an SSD. It's great! No need for an adapter necessarily, nowadays even cheap cases like my Thermaltake V3 ($50 case) have 2.5" HDD mounts.
S-ATA 3 drives are backwards compatible with S-ATA 2. They work as S-ATA 2 instead of 3 though.
If you have an HDD and no SSD, defiantly go for it. There are videos all over the net comparing SSD's against HDD's.
Here's are a few images giving you an idea of how much an SSD is faster than an HDD: