CALAMITA  I really like Yep's ability to search, organize, use tags and search based on any data in the file.   
(Version 1.8)

TMYERS22551  I bought Yep in last year's December MacUpdate Bundle. It was one of those "throw-ins" that I would have never bought or tried. It's now one of my favorites and I now consider Yep essential to my daily Mac experience.

Instead of making paper copies, I use the feature "print PDF copy to Yep" for just about everything that I want filed or managed. It automatically keeps track of all my PDF files, and if I drag a photo to Yep, it makes at PDF copy and tracks that too.

I have lots of files and photos that I use for my ancestry data and family history. Yep makes child's play of keeping that data organized for me.

I've been a Mac user since the days of the Mac Plus, and I have hundreds of programs that I've used. A few have become essential to my Mac software family. Yep has become one of my essential programs. For me, it has been stable, no observed problems with their updates, and it's feature rich. I don't endorse many programs, but I think Yep deserves at least a try.  
(Version 1.8)

KPS  All the previous versions of this app that I tried simply crashed every time it tried to import PDFs so the fact that version 1.7.9 hasn't just frozen solid yet is an improvement. Unfortunately, the app is still just as useless because it still will not import my entire collection of PDFs. It used to outright crash during import, now it just stops at roughly 13,000 and will not add any more. Since I now have over 60,000 PDFs it obviously isn't even coming close to cataloging them all. The auto-tagging feature is equally useless and failed to produce a good keyword cloud even from the document titles, let alone the content. I still find it easier to locate the document I need using Spotlight or the lowly "Find" feature in OSX.

I've always thought this app had lots of great ideas, but even after many updates I still can't get it to work as advertised. It's like a beautiful sports car that just stops moving after 1 mile -- pretty, but useless.  
(Version 1.7.9)

RUBAIYAT  This looks like just what I need. It is extremely frustrating that Apple doesn't support its own "Core" graphics format in iPhoto.

I have been converting most of my Postscript vector artwork and documents into pdfs and this may just let me organise and get a handle on them.

I ma just starting to use it so I hope that it will let me toss duplicates, quickly tag files and create smart folders.

It doesn't seem to have iPhoto's stack feature which is a shame, nor does it seem to let me access my Yep library through palettes in Pages or other cocoa apps that let you get at your photos in iPhoto. If it could do these 2 tricks I'd happily give it 10 stars, 5 a piece! s:-)  
(Version 1.7.9)

JACKWEB  I agree with JMAX, auto tagging doesn't seem to work. I thought it would actually search through for words within the currently selected document?? But it gives me random words which have nothing to do with the pdf I'm looking at? :(  
(Version 1.7.6)

KEYBORED  What was a pleasing novelty when it first emerged has become an indispensable program to those of us warehousing any significant number of PDFs in the chaos of folder hierarchies. It's especially useful for those of us who find or make our PDFs serendipitously in the middle of doing something else (and therefore can't always file them precisely and wisely), or for whom a PDF saved appropriately for one purpose becomes relevant to another one later on. Such is the life of writers, researchers, teachers, among others. For us, YEP has become the gateway to the riches buried in our HDs. Those who can remember everything without ever forgetting don't need it, of course. I do.  
(Version 1.7.5)

JMAX  I find Yep pretty buggy:

1. Unless I remember not to touch the name of the pdf while I'm tagging it, the tags won't be saved, and there's no warning to tell me this is happening - you just have to notice the doc is still in the "untagged" folder. If you're working quickly to get through a whole bunch at once, it can be pretty frustrating to look up and see some of them have to be done again.

2. Same thing's true when some tags get mysteriously combined into one, like "mac" and "camera" might be combined into a "mac camera" tag, which is worthless, and you have to go back and do it again.

3. There's the "auto-tag" function, which often doesn't work at all. That might be the good news, as when it does, a giant pile of mostly irrelevant tags gets dumped into the file. I suppose it doesn't really hurt anything, since you don't see the metadata later, but it's pretty weird and you wind up having to input the correct tags anyway.

4. Finally, the ability to select non-contiguous files by holding down the command key often doesn't work, and instead results in the prompt to the user: "are you sure you want to combine these PDF's?". Uh, no, actually, I just want to tag them the same way.

By the way, I've tried uninstalling and starting over, with the same results.  
(Version 1.7.5)

LEV  TexLogic - I originally took issue with Mendota (see my post above) but once he explained his thinking about caches and the naive user, I realise he has a very good point.

Your comment about it being the user's responsibility is fine but it assumes an ideal world. In the real world, there are lots of users who are barely potty-trained enough to get by, and apps that can help them will do better (and are more responsible IT citizens) than libertarian apps which just say "Don't whine if you get hosed". You might argue that Spotlight is redundant because it's the user's job to make sure that his/her files are consistently named and stored in a logical directory structure. Now tell Grandma when she can't find the first draft of her part-time PhD thesis...  
(Version 1.7.4)

LEV  Agreed with the previous comment -- no, Yep won't make your personal PDF files any MORE private than they already are. I don't think it's offered as a security solution. If Yep can see your PDFs, then, true, anyone can see them. And the originals will still be readable. It's more like a Finder for PDFs, and a brilliantly usable and slick one at that. You could perhaps imitate some of its features by using Spotlight and QuickLook in Leopard, but the tagging, sorting and finding functions in Yep are much more fluently thought-out. I find the application invaluable; and I have a feeling that no viewer/finder can apply a security Band-Aid to an insecure system. Security perhaps needs to happen at a deeper level and is in the hands of the user. But YMMV.

As to developer responsiveness: I only had cause to email them once, and got an immediate, and thorough, response.  
(Version 1.7.3)

MENDOTA  Yep, This won't keep your private files private.

This software claims to make important documents such as financial documents or medical records "more accessible." Unfortunately, this is true in more ways that the developers probably had in mind. The mini-images that Yep creates for your PDF files are all stored in the open in an unencrypted Library folder. How convenient for anyone who gains unintended access to your computer!

There are multiple ways the developers could address this problem, such as incorporating password protection to the cache folder, or allowing the user to setup a cache folder within an encrypted vault to pertain to the files within that vault.

I emailed the developers twice and never received a reply.

So while this is a nice program for browsing PDFs of articles, it seems like a poor choice for handling sensitive personal files. Eagle Filer or Records Master might be better in that respect, although they don't have quite as slick previewing and tagging, yet.  
(Version 1.7.2)