sata II, besides having theoretical faster speeds, also makes use of NCQ. This is native command queuing, which you can read up on somewhere else. It basically reorders commands given to the drive, so that drive locations are accessed sequentially rather than haphazardly, hence increasing efficiency. In reality, you will see only see subtle improvements, and more on server, or programs that need read/write access to large amounts of files across a drive.
For now, with drive mechanics and drive density pretty much unchanging, there will be no improvement unless you have multiple raid-0 arrays, wherein the extra bandwidth is enabled. But, if drive densities were to increase vastly, leading to faster transfer times, then that 300mb/s pipeline will be needed.
to be quite honest, i can't see it happening enough to come near reaching 300mb/s, but anyway, they will sell enough new mobo's and hdd for the gullible. When they have newer storage technologies (i think there is one that is perpindicular storage) then they probably will come out with a new interface for it, rendering sata2 obsolete.
edit: if you still want to make use of ncq, you can, seagate puts out a range of sata1 drives that have ncq. I have a 160GB model, but you will need a chipset that supports it, such as the nforce4. Note, that i don't have a mobo that supports it so i can't give you real world results.
ECS L7VTA mobo
2 x 512MB DDR400 Geil Value Ram (w/blue hs)
NVidia GeForce4 MX 420
Seagate Barracuda 120GB, 7200Rpm, 8MB buffer (pata),
Seagate Barracuda 160GB, 7200Rpm, 8MB buffer (sata-ncq)
BTC1108IM dvd writer (8X)
Aopen 1648 dvd-rom
sunbeam fanbus controller
Philips 190X5 19\'\' LCD monitor 12ms