I have been using Kaleidagraph for over 10 years. This week, I contacted support because text input no longer worked from AppleScript with the new all-Unicode AppleScript in Leopard (see http://developer.apple.com/releasenotes/AppleScript/RN-AppleScript/index.html).
They responded overnight and sent me the patched version 4.0.4 the next day. It works fine.
So - excellent support for removing bugs, even those not of their making.

Response to demands for new features is slower - I have been asking for improved scripting support for years. Wit IGOR, you get a much more powerful program, more math, more curve types, contour and 3D support with all the scripting support you might want but with a much steeper learning curve (2500 pp. of doc!) and for 2-1/2 times the price ($550.00 compared to 199.95 commercial for Kaleidagraph, or 395.00 vs 139.95 academic).

I use Kaleidagraph for 2D, IGOR for contour and 3D right now.