I've been switching between Things and Omnifocus as I try to pare down my setup. Neither one is really wonderful without the desktop software as well, which runs only on Macs. I found that Nirvana gave me some friction in both the iPhone app and in the web browser. I think it's fair to say that it's a niche player in the to do app market. If you are really committed to paper and just want to try electronic to do lists I'd recommend the Reminders app, Toodledo or Remember the Milk as inexpensive possibilities.