While this app is clearly designed for casual consumer use, it's actually a fully functional professional tool as well.
In my case, I have an artist that works in Manga Studio (Clip Studio Pro). Unfortunately, the English-language lettering tools in that software are a disaster (no kerning or ligatures at all, awkward, etc), so I need something else for lettering, and Comic Life is exactly the tool I need for that.
It supports advanced OpenType text features (auto ligatures, etc), is extremely fast and smooth-feeling, and tweaking the size of a balloon-shaped dialogue box is fast and easy. The sound effect lettering tools are also great--you can stretch, bend, tweak, and outline fully-editable text to your heart's content.
For my use, if I set up page sizes correctly, import full-page images, and set them to "actual size", I can then add lettering on top and export a 1:1 pixel-accurate PDF of each page with correctly anti-aliased text added. Works just as well in greyscale as color, too.
The only disadvantage to using it in this completely non-standard way: If you're not using ComicLife to do the panels themselves, clipping SFX or dialogue balloons to the edge of panels is awkward--it requires adding an invisible, borderless panel overlaying the one in the image and putting the text inside it. Functional, but clumsy, so I usually end up adding balloons in Manga Studio.
My only other complaints are that accessing the advanced text features is cumbersome, since most of it is buried in tiny tabs in a small floating properties window, and it's organized the way I'd expect. It's in there, but hard to find things if you don't know where they are, and they aren't always clearly labeled with what they do.
Also, it's a shame there's no "outline" feature to SFX text--if you add a border to text, the border ends up centered on the text outline, with no option to give it an external "halo". For, say, black floating text with a white outside border, this has the effect of making the text itself look really spindly if the border is thicker.