I've already moved my most of my 2008 Rival group to my winter/cx bike in preparation for the 2009 bits on my road bike. I'm upgrading mostly because of the shifter cable routing options (you can route both brake and derailleur cables on the same side of the bar, on either side of the bar) and the zero loss shifting for the FD. I will be using the Rival shifters with Force derailleurs.

If I were buying a full group, I would go with 2009 Rival without a doubt. For the price, nothing's even close.

That said, the 2008 Rival is awesome, too. If I were on an even tighter budget, I'd definitely pick up a 2008 group.