Which alternatives to Microsoft Excel are there for MacOS X and what are their (dis)advantages?
I would recommend you use Numbers from Apple. It is a very powerful tool, yet very easy to get started with.

You find more information about it on the Apple product page: http://www.apple.com/iwork/numbers/

Here are some hints for your requested features:


Filtering of tables http://www.apple.com/iwork/tutorials/#numbers-cell
Creating complex formulae http://www.apple.com/iwork/tutorials/#numbers-formula
Formatting cells http://www.apple.com/iwork/tutorials/#numbers-format


If you are interested in scripting Numbers, I highly recommend you to check out the AppleScript language guide.
http://developer.apple.com/library/mac/documentation/applescript/conceptual/applescriptlangguide/AppleScriptLanguageGuide.pdf

Numbers, as many other popular OSX applications, is fully scriptable using AppleScript.

My own experience with Numbers is great. I stopped using Excel a long time ago. I use it as my main spreadsheet application, and I found the learning curve is quite shallow. Although if you plan to become an expert user, it may take a while to adapt to the different environment. But as long as you go the "Apple way" by learning and using the built in tools, you will soon find yourself in a very comfortable spot.
A good example is the Address Book. Since I have started using the Address Book, mass mailing became very easy.