Although I've not used RocketDock in a long time, I don't think RD supports tabbed docks like OD Plus does.  But if you stack up OD free version against RD (which is also free), I think it's mainly a point of taste.  It skins a little differently...but it supports docklets like OD, and for the most part, OD backgrounds will work with RD.

Someone with more experience using them both might have better insight.

Rocket dock is a great little program. But if im not mistaken one thing it doesn't have which was a big factor in me getting OD was that it can not hide the taskbar. Correct me if im wrong. But I never find that in rocket dock and belvie me i tried:). Plus if you like the free version wait till you see that the other one can do. I love the fact that you can place docks anywere on the desktop that comes in handy for fetures like the clock and wheater dockets. Its more then worth the 20 dollers and thats coming from me. Im a hardcore penny pincher

The deal breaker with RocketDock for me was that it would occasionally not come back from autohide.  Even with no other windows open (ie. I'm staring at the desktop), and that's with the current version too (1.3.5).  I could never pinpoint when it would happen either... although it felt like it was usually when I opened Outlook.  The only way that I could get it to come back was by opening the Task Manager, selecting RocketDock and pressing Switch To.

I've never had that problem with ObjectDock though and about the only thing that I noticed that I initially missed from RocketDock, and I don't even care about anymore, is that it had more options for fonts (like colour, shading, etc).

I was just about to blame Vista... but then I realized that it did it to me on 3 different computers and 2 of them were XP.  Maybe it's just me then.