I installed Office 2011 for my wife for $10 through a home use program at work. I felt I had spent $10 too much. It was bloated. It added clutter to her dock. It took over file associations for everything under the sun. It corrupted fonts LibreOffice needed and I had to reinstall LibreOffice to get it working after installing Office 2011. The biggest headache was activation. Why should I have to activate an office suite? I can see activating some $50,000 computational fluid dynamics software but this is an office suite we're talking about here.
I don't like Office 2011. I don't like 2007 (or 2003 for that matter) on Windows either. I played with Office 2010 on a friend's PC and it had taken over the file association for OpenOffice (odt) text documents! What in the ?!?The last MS Office suite I liked was 97. Everything since then has added more bloat than features. Now I have to admit that back in the day, I hated Apple Quicktime on Windows because it came along and stole file associations for image files from other programs. I mean you installed it to watch mov files but it insisted on opening gif and jpg files. But the king of file association theft these days is Microsoft. On my wife's mini, when I restored the OS, it put back a trial of Office 2004. I then installed LibreOffice and you would think when she gets an xls or doc in an email it would open LibreOffice, right? No. She gets a popup that you can't run power pc apps on Lion because friggin' Office 2004 Trial is associated with all MS file formats. I had to drag the darn thing to the trash (good riddance) to get LibreOffice working for her.
I also own iWork and it does very well opening and saving Office docs. One caveat is there will be a handful of files that won't open properly. When I get one of those, between iWork and LibreOffice I get it open without resorting to MS software.
Start with LibreOffice. If you think you need more, try iWork. I would get Office 2011 only as a last resort.