For me the killer app for an iPad is reading and highlighting/annotating documents. The ability to download a bunch of PDFs (in my case, mostly corporate filing

For me the killer app for an iPad is reading and highlighting/annotating documents. The ability to download a bunch of PDFs (in my case, mostly corporate filings) into a Dropbox folder, and immediately sync that folder to the iPad so I can start reading and highlighting (with changes automatically saved back to Dropbox) has completely changed my workflow, making me a lot more efficient.