I've tried to make the switch to the iPad for everything and so far it is working well. My laptop was the main device for everything but I have been able to transition most of what I need my laptop for over to the iPad. So far, my laptop has not left the office once since I got my iPad at launch (I picked up a new one). There are a few critical apps and accessories that have made the magic happen.
Dropbox - all my work files are now in the cloud, so anything I do in the office I can get to on the road and at home
Quickoffice - being able to edit those files is critical. They just added the ability to edit PPT 2010 which was a huge hole in their feature set
Splashtop - The killer app. I can access my laptop on my iPad and I do mean the whole thing. I run straight up MS Office programs, Chrome, etc. from my iPad. They support external keyboards and the ability to turn the touchscreen of the iPad into your mouse control. This is THE app that makes it all possible because there are moments when the iPad can't do certain things.
Noteshelf - bye bye notepads, I really only do need my iPad for meetings.
Bluetooth keyboard - absolutely a must have if you are going to do any serious work or writing. I have the logitech made for iPad, works well.
Stylus - I have the bamboo, love it.
So what can't it do as well?
Media management - I still need a way to store and organize all the music, movies, photos, books, etc that I will consume on the iPad. Until cloud storage gets cheaper and faster, it will remain local on my laptop for now.
MS Office - this is the biggest hole but there are solid workarounds in place. Most of the apps I use are to gain access to MS Office, either via Quickoffice/Dropbox or Splashtop. Once MS releases Office for iPad, then a lot of this is no big deal.
Reply to an email with an attachment - it's plain silly that you can't simply attach a file from the iPad or Dropbox when replying to an email. Splashtop to the rescue, but really Apple? You can't make this part of the overall feature set?!?
But overall I think I am getting about 90% replacement out of my iPad. I'm a fan.