I think none is worth the price, but Apple's iWork and Doc to Go both offer a flexible pricing, by separating and in-app purchase respectively, so if you have high hope, you make less loss when you realize you can't get what you want.
I tried almost everyone of them and I still regret throwing 20 bucks away for quickoffice pro HD, not claiming others are better, just it's the most expensive while none of them meets my expectation.
If you mainly edit an make documents, I recommend Pages; if you mainly view slides, I recommend Smartoffice 2 cuz it's on sale every now and then and it dealt with PPTX pretty well with me.
If you wish to mainly deal with spreadsheets or make slides, I recommend forgetting it, none of them is tolerable IMHO.
My opinion is based on Microsoft office file types, I don't know how iWorks work with their original file types.
BTW, iWork convert MS Office files before opening it, that's a 3-5 seconds lag on average for medium length documents, if you intend to read a lot of one page word documents that might be disdurbing.
BTW, never even considering making word documents in Quickoffice, the typing lag is EPIC!