I must give several kudos to the crew at Openoffice.org. I don’t know who they are, but good job to them!
Being a Linux/FOSS fan, I adopted Openoffice.org (I’ve used it since it was an initial beta) as a strong replacement to MS Office. I actually own a copy of MS Office 2007 for certain things and prefer Oo.org by far. I’ve never used the database (Base) software and I’ve rarely used the Impress application, but Calc and Writer are my friends.
Calc, as an application, is stable and robust. I leave it up and running (not heavy spreadsheets admittedly–all several sheets with less than 100 rows and 100 columns) and it holds the grades for my four sections of courses that I teach at NC State. It has all (and more!) the functions I need and is quick. Due to it’s XML base, I export out of Google Docs (which my TAs and I use to coordinate grades) directly to OO.org. It opens Excel spreadsheets impeccably for me.
The Writer app serves and entirely different aspect for me. For any professional document that I would use, I use LaTeX as a typesetter of choice. However, Writer was invaluable to me when doing job searches. I could copy and past directly out of browsers to it and the format later as needed. I can export directly to .PDF (no crazy plugins) and further I can upload/download directly to/from Google Doc (notice I use Google Docs) a lot using the Openoffice formats. When mailing out job applications, my wife and I made multiple rounds of mailing and return address labels using OO.org. It is a fantastic product.
Another wonderful project (perhaps antiquate) although somewhat difficult to get runnin is the OOoLaTeX project. It is the FOSS sister to TeXPoint for Openoffice.org. However, it works for both Writer and Impress as opposed to just Excel in MS Office. Plus it’s free!