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)