I use Open Office and iWork on a Mac, and Open Office on Linux. On the Mac, Open Office's GUI sticks out as ugly and buggy. I think it's ok, but not great, on Linux, though. (This probably reflects that the Open Office port to Mac is still relatively new.)

I prefer Open Office Writer to Word because the way that it handles styles and document hierarchy make more sense to me. Also, the document layout seems to be relatively stable; at least I haven't observed many cascading layout catastrophes in Open Office Writer yet (as opposed to Word).

A big downside of Open Office Writer is its inability to embed many file types as figures (for example, you can't embed PDF). Apple's Pages is much better in this regard.

I dont use spreadsheets much, but even my light use brings Apple's Numbers to its knees. Open Office Calc is much better in this regard. However, Open Office's charts looks like crap, and Excel's chart feature is powerful but a pain to use. Numbers' charts are easy to use and quite pretty, but Numbers breaks down in flames for even moderately-sized documents (as in click, click, draw chart, go get coffee, come back in 5min, Numbers still not responding). I now crunch my data in Open Office Calc or Python and render down-sampled graphs with Numbers.

Open Office Impress is terrible. You are pretty much limited to bullet points, images, and ugly transitions. I hate PowerPoint. Keynote is great; there is no way I am ever going back to a presentation package without a "magic move" transition.