Ehhm, I like mouse aim in levels that were explicitly designed with it's versatility in mind. In regular multiplayer and classic SP I rarely use it. Having both hands on my keyboard is one of the pros of playing JJ2 after coming back from actively moving my right hand in Counter-Strike.

In terms of 'comfort zones', well you have to properly configure and get used to mouse aiming if you are to complete levels such as Ozymandius and Rocket Powered Rabbits, as poor aiming/movement means death in those levels. To me it was a fun experience completing them, and you know I voted Ozymandius and Decimation as favorites in last 2 JCS awards!

While Cooba has a point on the learning curve which I believe shouldn't hold you back from trying new kinds of levels, I don't fully agree that all-keyboard play is obsolete now and there's clearly a golden mean here which I've gratefully adopted.