There are lots of factors to consider before investing large-ish amounts of money in software. Freeware is increasingly viable for many users. I use ENVI primarily because it is used in teaching at my place of work. ENVI is, I believe, very simple, even pedagogic and lends itself well to teaching undergraduates. What is more, ITTVIS have great offers for Educational Users.
Another reason I like ENVI are that an ever improving range of specialist, high-end modules are available to extend the platform. A further reason to choose ENVI became apparent today: a colleague said she loved ENVI because the Support team are so prompt and efficient. Not a trivial reason to help inform your software choice.