I remember looking at this a few years ago. My main reason for not using it was the time it would have taken to tag all my files.
Basically, if you have so many files that it's worth tagging them, you need to spend a lot of time tagging, and if you have so few that it's reasonable to tag them all, you don't need this system. If you have a manageable number but expect tagging to be useful in the near future, then this might be useful.
I'd be interested in hearing from people who have found a use for TMSU.
Throwaway, but I've used it to organize downloaded porn. Downloaded videos tend to have terrible filenames, but reasonable titles or tags on the website. I have some scripts to scan through the filenames and automatically apply tags, but for the most part it still goes unused. It's a fun idea though.
A number of years ago my media filenames were getting out of hand and after a drive crash I decided to start naming all media files manually with a basic structure. It worked quite well and I've kept to the same basic idea for a number of years, with more than 10k files named this this way when initially saving the file.
It follows a fairly loose mental structure: Broad Category - [Sub-category -] [Series Title -] Name/Media Title (Creator, Source, Short Description/Tags)
The basic idea has been to organize the filenames by broad to narrower info from left-to-right, which helps with quick filtering/searching and allows many categories of files to be kept in large single directories while being readable. By adding the creator and site where needed it also allows me to trace the source. Not a perfect naming system but it only takes a little longer to save while adding considerable value long-term.
I have a library of a lot of ebooks about a lot of subjects, I order them by folders, but is very common to have interesections on the topics they cover. I could tag all them with the folder name, and then manually tag the subtopics each book covers.
edit: too bad it isn't on ubuntu repositories. I've a personal policy to avoid programs outside the repositories
Not only can you have multiple tags, you can have icons which are conditionally displayed based on tags. Tags are stored in a single metadata file, one directory per ebook, so can be parsed with a script when performing bulk operations.
You can then customize search engine like recoll to display ebook metadata as contextual snippets in full-text search results. Best of both worlds.
A lot of files that end up on our machines come from the internet, and, at least on OSX (in most browsers), downloaded files are tagged with origin URLs [0].
It's a small step from there to a companion tool that crawls your filesystem for URLs, and attempts to classify them based on a keyword analysis of the origin.
For similar reasons, I just can't bring myself to use OSX's tagging feature; it just seems like so much effort without a good use case. I'm not sure it helps that the defaults are simply colours, rather than real examples. And I don't really want to think about how files are shared (server, dropbox, google drive, usb stick) and do all the necessary research into how tags work in each and every case. I too would be interested to hear if anyone's successfully found an effective use for file-based tagging.
OS X tagging is sometimes handy for tasks over a short time frame. Picking out photos to upload to FB? Scan through them all and hit cmd-6 on the ones you want, maybe cmd-7 on the ones you need to crop. Sure you could use Bridge, Aperture, etc. but having it built in to the OS is nice.
I rely heavily on spotlight to find files I want, and about 99% of the time it works perfectly. Sometimes there is a file I am going to keep that I suspect will be hard to track down using spotlight in the future; either because it has to be badly named (source code files), will not be easily indexible (scanned pdfs or images), or my interest in it is not directly connected to the content of the document (i.e. a description of an algorithm with an obscure name which I have a specific application in mind for).
In these cases I use the tags feature to augment the metadata so that I am pretty confident I will be able to find it quickly in future, even if I can't remember exactly what it was or why it was originally saved.
I tag all of my Firefox bookmarks. I've been meaning to export them out of Firefox and use some other kind of bookmark manager or write my own (perhaps integrating it in to Emacs).
Something like this might help, if I make each bookmark in to a separate file.
Basically, if you have so many files that it's worth tagging them, you need to spend a lot of time tagging, and if you have so few that it's reasonable to tag them all, you don't need this system. If you have a manageable number but expect tagging to be useful in the near future, then this might be useful.
I'd be interested in hearing from people who have found a use for TMSU.