ZEBA  How does this compare to Pro Fit lately?  
(Version 6.04)

ATRAINDD  Amazing software at a very fair price. I came from a background of C++ and found Igor was very simple to pick up and extremely flexible. The support is great and the manual is extremely thorough, walking you through using the GUI and through advanced programming.  
(Version 6.01)

TOMALYZER  I've been using this software for about six months now, and I'm yet to find fault with it.

The hybrid interface is fantastic, it lets you see exactly what the program is doing, to the extent that you can easily replicate any sequence of operations from the GUI in a procedure - without the unreliable and awkward script-recording found in other programs.

The interface (on both Mac and Windows) is blazing fast, clear and well designed. Graphs are almost publication-quality out of the box, and can be tweaked with extreme precision. It is sufficiently fast to run well on older computers.

I've yet to find anything that Igor cannot do. Get me a coffee maker with a USB socket, and I'll get it to make coffee!

Igor is insanely under priced and undiscovered considering it's capabilities and quality.

The one thing I found necessary was to spend some time building a set of procedures to do some fairly basic stuff that could be done out of the box. The way I work involves manipulating windows full of waves, and I had to write a couple of scripts to manipulate everything in a window at once - I don't think it would be too hard to include this kind of thing in the main distribution. Also, it's worth bearing in mind that if you want to deal with external hardware you'll need Igor's XOP toolkit extension, which I thought to be a little over-priced.  
(Version 6.0)

DOMINIKHOFFMANN  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.  
(Version 6.0)

TORYALAI HART  Igor is the way. The program is fast, the language is easy to learn since menu options generate command line equivalents that can be copied into scripts, which can in turn be made into functions. If you work on large datasets with similar structures, Igor will make the impossible feasible through incredible job batching power. I can't remember the last time it crashed - and sometimes, I have it working for days. Even the GUI generation system is friendly and very intuitive. Besides what often seems like unlimited flexibility, the wavemetrics team is always available and incredibly responsive to user needs. I've never worked with a company that showed so much dedication to support, free of cost. They have a great mailing list and good documentation. 3 years of using it: no complaints, only great surprises and continued amazement at how I am able to evolve the Igor environment to suit my needs.  
(Version 4.0.7)