I don't think that was what the OP was asking.
No one doubts that MS Office will be around for ever and a day.
OP:
Can you now use iWork 09 instead of MS Office?
For me, the answer is yes.
But then again my needs are pretty simple. No huge spreadsheets or mass mail merges to '000s.
And Keynote is a lovely lovely program.
I use to think so as well, but ever since I moved the bulk of my work over to Windows and have forced myself to use Powerpoint instead of Keynote - I'm not so sure. Of course very simple, visually arresting results are Keynote's fortes... but in terms of incorporating information from other sources and also in terms of ultimate presentation flexibility, I'm having to give considerable props to Powerpoint.
It's an interesting result, because I started putting together a lot more presentations myself when I started using Keynote, whereas before I used to sit in front of a Powerpoint window and think 'WTF do I do now?' far, far more often. However once I'd attained a certain degree of presentation-fu as it were with Keynote, I found that Powerpoint ultimately does a better job.
I give myself the option by being able to use both iWork and Office (Mac & PC), but if I had to choose one suite it would definitely be Office under Windows.
Like I see much of Apple software, it's turned out to be a bit like a set of training wheels. Somewhat basic but gets the job done in a very pretty way by putting more power into the hands of the relatively clueless starter - and ends up, if you are a learner, preparing you for less easy to get into stuff which delivers superior results. Whether people make that switch, having donned the training wheels is... well, your call. Even mine was kind of forced, as if it weren't for machine logistics problems I would have stuck with Keynote.
As for the other components, well I have Word-fu and Excel-fu aplenty. For that reason, Pages is just a useful jotter / infosheet maker for quick and nice results(once again the Keynote / Powerpoint comparison), and Numbers - never used.