I own a laptop, an iPad, and a Kindle (2nd generation). They all have their own strengths and weaknesses. If you want to do quick reference, need the ability to switch between multiple pdfs easily (especially when using them for character or mission design), or need a guarantee that all of your PDFs will be readable, the is no substitute for a laptop or netbook. If you want low cost, maximum portability (low size and weight, and amazing battery life), then you want a Kindle or Nook. However, you should be prepared for display issues with tables and graphics, and zooming/panning may be annoying.
An iPad will be a middle ground in both price a capability. Almost all PDFs will display properly, and zooming will be better than on a Kindle. But it won't match the Kindle's portability, and the touch screen interface is slower and more klunky for thing like typing, text selection, or going to bookmarks (the Kindle has a mini qwerty keyboard and directional input device). The screen is nice, but not full sized, so you'll still have to pan and zoom. The iPad is basically a big compromise between the other options.