Welcome to the Forum. I hope you find out answers to all your questions here. It's a great place to 'hang out' and 'meet' with other iPad enthusiasts. I usually end up finding out more about my iPad than I really wanted to know!!!
I think you'll get a variety of answers to your question. People have a wide range of uses for their iPad - some use it mainly for entertainment, others as part of their profession.
For my part it's a great replacement for many of the tasks that I previously carried out on my PC. So I use it to prepare and review documents.
For that I use Apple's 'Pages' (MS Word replacement)
For spreadsheets, Apple's 'Numbers'
For presentations, Apple's 'Keynote'.
These are 'stripped down' versions of the Mac iWorks Office suite, so don't expect full 100% compatibility. There are several other third party Office suite apps, of course, but - more or less - they all have similar limitations.
Very complex Office (Word, Excel, PowerPoint) documents may not render 100% correctly on the iPad. But, in my experience, documents that you originate on the iPad do render 100% correctly when you migrate them to the PC.
I use Informant HD as my diary/tasks/event/project manager. It's a relatively complex program but its great advantage (to me at least) is that it sits 'on top' of the iPad's native Calendar app and imports any appointments that appear in it. Since the iPad can receive meeting requests, appointments etc via, say, email and these are, by default, migrated to the native Calendar app, it's a great bonus that I don't have to manually copy these into Informant. Another advantage of Informant HD is that, unlike many apps, there's a comprehensive manual...
You'll almost certainly know that the iPad doesn't support Flash, so websites that make extensive use of embedded Flash are not really accessible to the iPad. This is probably the most endlessly discussed topic on this Forum, so I won't say anything about this...use the Forum 'Search' button near the top of the web page if you want to find out more....
I like the 'TuneIn' Internet radio app for keeping in touch with the world while travelling. It's unbelievable value for money and gets consistently good reviews.
Most iPad users will choose GoodReader as a must-have app. It's an excellent file manager, offers easy ways to migrate files to/from your PC without the use of iTunes, and provides a very valuable PDF file markup/annotation facility.
OK - I've gone on far too long. Enjoy your iPad. I'm sure you'll get lots of replies to your post.
Tim
Scotland