I have both installed on my MBP (penryn) and, so far, I have not seen much difference in speed between them. Probably I'll see it only with huge files, but opening 200 Mb plus PP presentations is more or less the same.
The reason I have both is because I need to use Endnote with Word for my work. I don't exactly know the reason, but the Endnote version that I have (Endnote X) does not seem to work well with 2008, so I have to open word 2004 if I want to use it.
Yes, it's a frustrating omission (especially if you use Excel for any kind of scientific or technical graphing), and I can't see why such a feature would be taken out. I've submitted feedback to Microsoft asking for it to be put back in. If you read the Mac forum over at arstechnica.com, two of the developers from the Macintosh Business Unit at Microsoft post there and have been very helpful in answering a lot of questions (and complaints) that people have had over Office 2008.
I'm writing my PhD at the moment in cell biology, so lots of error bars for me too 😉
I found overall little difference between Office 2004 and 2008 in terms of speed. Certainly not the huge boost I was expecting. PowerPoint is probably the most noticeably improved. The startup times for the Office 2008 apps are terrible. I haven't timed them, but it would not surprise me if Office 2004 apps (under Rosetta) start up in the same amount of time. I ended up deleting it from my hard drive after about 2 weeks. Fortunately my department has a site license for Microsoft products so I didn't need to pay for it. I would have been angry if I had. As I mentioned above, there are some good threads on the arstechnica.com Mac discussion forum about Office 2008, with some of the Microsoft developers posting responses. The overall response from the Mac users over there to Office 2008 is not good. I'd agree.