Yes, it is worth it, but only if you're not buying iLife '09 (which has iMovie '09)
This is because iMovie '08 has limited editing capabilities, it is more of a drag-n-drop editing program. It is easier to use as well.
iMovie 6 HD has video & audio effects, which are very useful and fun to use. It is a little more complicated, but not much more. If you are planning on using iMovie 6 HD in conjunction with iMovie 08, it is a bit annoying to transfer the 06 movie to 08. But for me, the only reason for doing this would be to be able to use iLife 08 sound effects.
iMovie '09 has video effects, but they are more of just whole clip effects. By this I mean that iMovie 6 HD has things like lightning bolts, stardust effects, etc. that can be put into the movie whereas 09 is more like aged film, black & white, sepia, etc. Also, you can use plugins in 6 HD to get more effects.
The aspect of iMovie '09 that is compelling me to buy it is its video stabilization feature. It can stabilize shaky videos and make them look professional.
I realize thats a lot of stuff to consider, but hopefully it helps.
Good luck with whatever you choose.