I've used Office for the longest time on Windows and for about a year on Mac (Excel, Word, Powerpoint & Access), and in terms of word editing, presentations and simple Excel tasks (such as accounting, simple math stuff) iWork beats the crap out of Office in my opinion. There's some minor flaws, but the visually pleasing result you get from using iWork over Office more than makes up for those flaws.
The reason I prefer iWork is because you can make everything look highly professional (like you hired someone to do the layout for you) like you might have noticed. My first three years of university I used Office,a and never received a compliment on my slides or my reports (and not to brag, but I probably had the best looking slides and reports in the class).
Once I started using Keynote for my and my groups' presentations, the profs and the other students were simply blown away. They would literally say 'wow' during the presentations. When I got my reports back, the profs would always comment how professional the report looked like, never happened while using Word. Once I was even asked if I used a graphics designer to format my reports! (anything that looks remotely nice is a rarity in management schools as Word doesn't exactly lend itself well to that)
And being able to export Pages documents as a pdf without expensive add-ons is also great. Ensures that your document can be opened by anyone and still look the same regardless of OS or the word editing software they use.
The only drawback I have with iWork is when you're working in groups and might juggle multiple files that needs to be combined there's the possibility of issues regarding interoperability between iWork/Office/OpenOffice. (Personally don't have much experience going between these three so I don't know for sure).However, after Google Docs came around, we would use that as it's much more collaborative. Once we had a final version on Google Docs, I would copy everything from there into Pages/Numbers and do the final editing and layout there.
I don't know how large files you're working with, but when I've worked with 20 page newsletter with lots of large images I've had no trouble. Just remember to save frequently. Maybe I'm wrong, but I didn't discover an auto-save feature like you have in Office.
Also, if you do really complicated stuff in spreadsheets that require more advanced Excel features normally, you might want to check and see if Numbers has that feature also before making your decision.
I'm definitely never using Office again though. Keynote alone makes it worth to get iWork if you ever have to do presentations.d
