If you have to ask the question it is likely because you don't need the high end features available in the MS Office suite. Most people don't, but the bloat is there (and is obvious) whether you need these features or not.
This has been said by others as well, but from a cost per feature perspective, iWork hits the sweet spot for most users. Myself, I have both (I know, sounds stupid, but I need office for compatibility with work documents, and I use iWork for everything else).
Pages is simply nicer to use than word, and unless I need some of the esoteric features in word (or I need _precise_ compatibility) I do most of my work in pages.
Numbers can produce some really beautiful results, but in general is pretty lightweight from a feature perspective. Probably too light weight. Happily I almost never need to _produce_ complex spreadsheets, so I can easily use Numbers for everything I need it for. I have Excel for those hairy spreadsheets my boss is so fond of sending me.
Keynote blows away Powerpoint. I have never found a reason I would ever use Powerpoint instead of Keynote to produce a presentation. But then I've never liked Powerpoint, even when its the only option I have.
There is one other products you may also want to consider:
Look into Omnigraffle instead of Visio. Visio was a nice tool before Microsoft got their hands on it (early 90's? Can't exactly remember when). Now it's just awful.