I've been adding some category-type tags to books I post and now I wonder if I'm wasting my time, if I'm the only one that sees them LOL!!

No, you aren't the only one who sees them.  They will be there to be seen by anyone who looks at the same page the reviews are on. You are the only one who can delete them.  It is a great idea to add category tags to books.  If it is a series you could also add the number of the book in that series. 

Just use them appropriately please! I have seen tags that say "On my reminder list" - this is not a personal sticky note as all will see it. 

Actually tags can be used as personal sticky notes.  The rules say that you can use them however you choose.  I have marked all of my books on my TBR pile so I will know when browsing others bookshelves, that I already have that book.  Only the most popular tags will stay with the books, so eventually your personal notes will fall off and no one but you will see them.

I finally marked all my RL's.  That way when I'm perusing someones shelf on book bazaar I can see easier if they have more than one.  I haven't memorized them all.  As more and more people tag books only the most common tags will be seen by everyone. The more personal ones will be seen less and less and some only by the person who created the tag. You will always see your own tags. 

You can tell if it is your tag or someone else's by the little red X next to the tag. if it is yours the X will be there.

I also just tagged all of my wish list and tbr titles. I am really bad about requesting a book I already have or that I've recently read and sent on to a new home. When I was tagging my current lists I discovered I not only had a title on a BOB request but I had a copy in the mail to me and had recently mailed a copy of the title. I was able to cancel the duplicate request but I now have a book coming to me that I've already read. So I will probably turn right around and repost it.

What I am doing is removing the tag once it arrives. For example:

I do a BOB request and am waiting for a response, I tag the book: BOB - requested.

Once the BOB is accepted and mailed, I change the tag to: BOB - In Transit. At this time I remove the old tag.

Upon receipt of the BOB, I change the tag to: Library - TBR, once again removing the prior tag.

If it was a book on my Reminder List then I had it tagged Library - Wish. Once I've requested it I go and remove both the tag and the title from my Reminder List.

Sadly it is this list that has gotten me in so much trouble with duplicates and triplicates. I frequently fail to remove the title once I have received it and then I'm going through my list looking for a couple titles to order and reorder it.

I'm very happy with the tag system.

Naomi

Actually, there is no "appropriate" way to use them.  You can use them any way you want.  From the help center (bolding and underlining added by me)

Book Tags allow you to mark books any way you like. 

• You can tag a book as "great" or "why would anyone read this?"
• You can apply any number of tags to the same book.
• You can view all the items in the database bearing a particular tag by clicking on the hyperlinked tag wherever it appears on a book listing.
• Others can see your tags.
• When you view a book you will see ALL of the tags you have applied to it, as well as the most commonly-applied tags for the whole club.  Less-commonly applied tags will not necessarily show, especially if a book has had many different tags applied to it.

• You can tag books for yourself if you like: "ordered for aunt martha" or "I hated this author"
• You will see the tag when you view the book on your Bookshelf, in the database etc.
• We will soon have a way to view tags on the site so that you can use them as another way to "search" for books you might be interested in reading.

• Maybe.  Others will see your tag on a book listing if it is one of the top-15 tags applied to the book or if they have also applied that tag to the book; otherwise others will not see your tag.
• You will always see your tags in the tag list on a book, even if you are the only one who has tagged the book that way.

Also tags are only for that ISBN.  So if you tag a hardback version of a book the tag won't appear on the paperback version for example

Sometimes, if it's an obscure book, it helps to tag it into a genre or sub-genre.  I've had a couple get requested off my shelf shortly after doing that.