A friend of mine uses PDF as the default format for saving Photshop files. For both master and working documents. He never saves anything as a PSD. Is this good practice?
Actually, you can save as PDF with Photoshop Editing Capabilities checked and retain most items.
The danger is, if as a PDF the file is opened in any other application and edited (or even saved) you may lose things without realizing it.
So, while it's certainly possible. It seems there's no valid reason to do so. I personally prefer double-clicking a Photoshop file and having it open in Photoshop, not Acrobat. I also need to keep track of actual PDF files compared to Photoshop files. How would one know the differences between a Photoshop PDF, Illustrator PDF, Indesign PDF, etc. It seems pure folly (or inexperience) to me.
In some cases people are just different to be different.
In general, as Scott says, it doesn't make a lot of sense to save PSDs as Photoshop PDFs. There is one important exception: if the file will be used in a layout (InDesign, for example) destined for print, and it contains vector information such as shape layers or type, then it should be saved in PDF format and placed in the layout as a PDF.
The reason this is a best practice is that when a PSD is placed in a layout program such as InDesign, what is actually placed is a rasterized composite. All vector art (at unlimited ppi) becomes raster art at whatever the image resolution happens to be at output, which would typically be 300 ppi for print. There is a world of difference between 300 ppi text and the 2800 dpi generated by an imagesetter or platesetter. A placed PDF retains vector information as vectors in the layout and when it is exported for press.
One of the more useful "minor" improvements in (if I recall correctly) InDesign CS5 was a change in the context menu to add "Edit Original with..." so that one could choose which program to use to open the linked file. A Photoshop PDF can then be opened in Photoshop for editing for a fast round-trip.
...uh, if you think it's good practice to take a photograph of a letter you just typed instead of saving it as a text file, sure.
If you save your work as a PDF, it's flattened. You can't edit it. You can't alter it. (Well, you can, but with some difficulty.) You can't change the layers, the type, the effects, anything. You practically have to start from scratch.
I am struggling to think of an instance where saving a Photoshop master file as a PDF could ever be useful.
Often times I will save a master file as both an editable photoshop PDF backup and a PSD. I work on my files on a network drive where is is useful for others to be able to view it and proof the file before I export the final version flattened. It's useful in this case for others to be able to view the file in acrobat as it's being worked on without having to export it a hundred times for each iteration, and it's also useful to be able to preview the file in acrobat without having to wait until photoshop loads up to take a peek at it.
However, if saving as a master file, I always make sure to denote this in the file name (PSMaster) or some sort as it is easy for someone to accidentally flatten a pdf-saved master if you're not careful.