I personally have no problem with nonvisual Watermarks. The only people who see them are people specifically looking for them, and they do not limit the abilities of the purchaser. I have quite a few books from O'Reilly, and they have watermarks. I cannot see them, and I am not limited in what I do with them. However, there is a bit of an understanding, to where I know that if I give my books to someone else, or put them online, O'Reilly could track them back to me. Keeps me honest should I decide to do something not so honest, but if that thought never crosses my mind, neither does the one that they've done anything special to the book.
That said, I hate the visual watermarks, that cover portions of the page or images, even if they are semi transparent. They are annoying, and prohibit immersion.
On traditional DRM, I am severely limited in what OS I use, what devices I use, how many devices I use, sometimes how long I use it, etc. Basically, I, the legitimate customer, gets hamstrung while the people who pirate the content have none of these limitations. I've had it where the DRM has prevented me from playing a game I paid for, the developer refused to fix the issue with the DRM (which effected a decent sized subset of the customers), and ultimately I had to get a cracked copy of the game in order to play it. It is madness to think that they have no concern for hurting the legitimate, in their blind, vain, attempt to stop the pirates.