Metasynth is a truly unique application. Chances are good that you've heard of its claim to fame: the odd 90's era IDM gimmick of converting pictures into sound and embedding the resulting unearthly noise in music. Can it do that? Yes! But that's far from its only value. Metasynth is a rich, complex audio development environment with many layers of rich functionality and a distinct personality.
The Image Synth alone -- Metasynth's best-known asset -- is worth the price of entry by itself. The fact that it can convert an image to sound is due to the incredible flexibility of its visual to audio mapping. If you think about it, the piano roll in every DAW is also an image-to-sound engine in a way! The great insight of Metasynth's developers is that there is no qualitative difference between a piano roll and a spectrograph; both are ways of mapping vertical space into pitch and horizontal space to time.
Fundamentally the Image Synth converts a grid of pixels via a mapping into sound, using a choice of any of several synthesis methods. The fact that the vertical mapping can represent anything from a whole-tone scale to the finest-grained microtonal scale, or anything in between, including user-defined mappings, makes the image room a tool that lends itself equally well to crafting musical phrases, synthesizing new instruments, and developing previously unheard soundscapes - and blurring the line between all of these.
Metasynth's functionality has also grown and evolved over the years: the Effects tool provides a number of sound editing and mangling options, from simple delays and reverbs to complex granular and spectral weirdness, all integrated with a unique envelope editing system unlike anything I've seen in another audio app. The Image Filter uses the same image-to-audio mapping used for composition and synthesis in the Image Synth tool to implement an incredibly complex time-varying multi-band filter. The Spectrum tool (which could easily be marketed as a separate app worthy of investigation by itself) provides a way to break down and recombine the spectrum of a given sound and use it for creative re-synthesis. And the Montage and Sequencer tools provide just enough multi-track editing / mixdown and bread-and-butter note sequencing functionality, respectively, that you can write whole tracks end to end in Metasynth without ever reaching for a separate DAW, if your heart so desires.
That isn't to say that Metasynth is the right choice for everyone: its design is idiosyncratic, absolutely the brainchild of a specific vision integrating the visual and audio realms in a unique way. Some basic UI affordances most of us take for granted, like multi-level undo and ubiquitous context menus, are missing, and some parts of the app like the Montage room, although handy, are a little underdeveloped. Most importantly, although you can get good (and very weird) results without so much as cracking the startup guide, truly mastering the app and adapting to its quirky personality takes a significant investment of time and mental energy. But for the musician or sound engineer committed to exploration, it is an investment well worth making, and will pay off for years to come.