Office '08 has a lot of nice features, but iWork '09 is a lot 'leaner'. You'll notice a big difference between the two suites.
Personally, I like iWork '09 a lot better than Office '08 (own both). Word '08 has crashed a few times on my wife and I, but Pages has crashed maybe once that I can remember.
However, my wife uses iWork exclusively now that iWork supports Mathtype (math professor), and hates Office '08. Sometimes students will send her assignments in .DOCX format (Word '07) that will take upwards of 2min to open in Office '08 (on iMac 2.6Ghz 4GB RAM); however, if the document is sent in .DOC format, it opens fine. I contacted Micro$oft about it and they said it is a 'known issue' because of the font-checking that goes on w/ .DOCX format (that's what they said). Also, if a student enters an equation in Equation Editor (Word '03 or '07), they will not show up in Word '08--only placeholders--another 'known issue'. However, Pages opens .DOCX files just fine (very fast), but the equations have a 50/50 chance of showing.