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It's remarkable to me that people seem to think that Adobe Photoshop is an amazing product.It doesn't have to be perfect inside to be an amazing product. People could care less about how PSD is structured or if the code is a spaghetti mess. The organic growth is also inevitable when you have something that evolved over 20+ years and want to keep backward compatibility as much as you can --including with tons of external third party tools. It's not like they are idiots and can't get it right if they are given the chance to do it from scratch.
The thing it: it works, it works great, and nothing compares to it in it's field.
For the actual graphic designers, photographers, illustrators, etc that use it, there is no alternative that is as capable. There is also no alternative that can be integrated with a full ecosystem of products, from TV/Cinema work (Premiere) to print work (InDesign/Acrobat), to Web (Flash/Edge/Fireworks), to Vector (Illustrator), with all being leaders (or almost) in their particular field.
I'm talking for professional workflows. For just getting some pics up to a webpage, without any chance of it be used in other contexts, YMMV.
Compared to GIMP for example, there are tons of stuff that make it better for a professional workflow. From better text support, to a complete RAW file import/edit support, to a complete (and working for years before Gimp got any) CMYK support with gamut warnings and the whole kitchen sync, to Smart Objects, to vector support, etc.
Though, latest releases and transitions (like to 64bit Win/Mac, to Cocoa, to the new UI etc), seem to have given Adobe also chances to rework parts of the code.