Your opinion may vary.

Personally I like Keyclone - It's extremely simple and easy to use, took me 10 minutes to figure out how it worked when I first downloaded it forever ago, I paid once, and it's mine forever. Rob also provides ridiculously excellent support.

ISBoxer, while it has a ton of great features, is not nearly as intuitive to start, has a subscription fee, and for me just didn't 'mesh'

However, if I ever started melee multiboxing, I could see having to switch to ISBoxer, as it has many elegant solutions to the problems of melee multiboxing.