I can wholeheartedly second every aspect of Toryalai Heart's review.
In addition, Igor is the only graphing package I am aware of that allows me to fine-tune the layout of a graph presenting scientific data to journal-quality standards. Unfortunately, because of the prevalence of Excel, a lot of data these days gets presented in aesthetically displeasing laid-out graphs. Igor, however, gives the same control back to the user scientists had back in the day when they would send data to specialized graphic artists, before submitting to a journal.
My rating of five for "Ease of Use" has to be understood in context. This does not mean that the program is easy to use by someone who has never done any data analysis or built a graph from given data. Rather, it represents the logical, makes-sense, connection between the way Igor uses the dialogs of the OS's GUI to build the commands of its programming language. Once one has built those commands, they are easy to tweak on the language level and then re-executed, which saves a lot of time over having to re-specify parameters modified only slightly, via the GUI.