if you have a multi core / multi proc system - it's worth it,
If you have any desire to use nested projects - It's (DEFINATELY) worth it.
if you don't use either of these things - then it might not be worth it. If you do anything with more than 2 cam multi cam edits - V6 makes it much easier with either Ultimate S 2.0 or excalibur 4.5. (Ultimate S 2.0 being my recomendation because of the the ability to add on Grafpaks so easliy etc...)
Nested projects rule. I use them all the time now including PTT projects. Drag and drop everything I want on the time line and do the print. Very easy to do.
If you have "Show active take info in events" enabled, then accidentally move video away from its audio or vice versa Vegas will display in RED the amount that one is out of synch with the other.
You can then right click the one you want to move and re-synchronise it with a single click, either by Moving (the event moves back into synch) or by Slipping (the event stays where it is, but content slips back into synch.)
And 60i to 24p conversion is definitely superior in 6. I upgraded from 4 but I believe this has been the experience with those who upgraded from 5 also.
Nested projects are a new great way to work. Do all my projects that way now.
Media Manager allows you to create libraries for clips, music , sound effects etc. I usually have it turned off, but when I need it I click the tab, turn it on and search for appropriate media. For me it's a great time-saver -- not perfect -- but great for quickly previewing sounds, clips, etc.
At least from v4 to 6 I have better success viewing the video on my TV through the DV camera than with v4. Almost flawless. With 4 I would get a lot of disconnects and then reconnects of the firewire.