I went from 3.5 to 3.9 recently, I'd say it's worth it as all the stuff I used to have to patch in from 3.0 is just there and works nicely - Amidock is nice, the (registered) Genesis TCP/IP stack is also nice, the unpacker has saved me time, I like the glowicons, the occational need to look at the manual is easy as it's all in HTML on the CD ... and so on. Oh, and I'm warming to the Class Action file manager that comes with it too.
Yes it can be done from Aminet, but it's saved me more than the puchase price in terms of my time to get the machine set up right alone.