Not worth sending in the rebate. You are better off waiting for the official release of Office 2k8 in a couple of weeks.
Main reason is waiting forever for the fulfillment CD/DVD to arrive. The prices between 2k4 and 2k8 are about the same. Especially if you have 2k4 on an intel mac. 2k4 runs very slow on intel macs. 2k8 will be optimized for intel mac. So it's pointless having 2k4.
I bought a Gateway laptop with Windows XP SP2 last January (just 3 weeks before the official Vista release). I was also promised a Vista upgrade DVD. Guess how long you have to wait....well I waited until the beginning of May 2007 for my upgrade DVD to arrive. Vista was released at the end of Jan. 2007.
So since Office 2k8 supposedly runs much faster on intel macs, it's worth your time to wait for the official release rather than have to wait 3-4 months for your upgrade DVD to come in via mail.
iWork 08 is fine for about 80 percent of consumers out there. It looks pretty (keynote 4 is far superior to powerpoint) while Pages 3 and Numbers v. 1 are not as powerful as Microsoft's Word or Excel. There is no Outlook program in Mac Office 2k8 (they have Entourage which works like Outlook but you cannot synch with MS Exchange with Entourage...you can sync with Outlook). So unless you are a corporate user, iWork 08 is a fine alternative. iWork's files can also read Microsoft Office 2007 (for windows) new formats also.