For me, Soft DRM is a viable option that helps the book provider to identify who they provided the book for/to. I'm not aware of any formal analysis, or reports, of how Soft DRM fares in terms of things like unauthorized copying. I've found 'soft copies' of books online that were watermarked for someone else so unauthorized copies/distribution exists. However cracked (most likely unathorized) copies of 'hard' DRM books are also available on the internet.
In terms of my own purchasing practices, in 2013 I personally bought at least 50 ebooks online. These were mainly technical books. I bought the vast majority from 3 publishers: O'Reilly, Pragmatic Programmers, and Packt Publishing. They are all DRM Free.
http://pragprog.com/frequently-asked-questions/ebooks (they say most, not all books are DRM free. The ones I've bought from them, at least 10 titles, all are).
I have bought various books that have soft DRM e.g. from Apress http://www.apress.com/customer-support/ and Commonsware https://wares.commonsware.com/
And I've bought very few books with active DRM, with one exception - ebooks bought for Kindles. I explain some of my reasons later on in this reply.
Here's my original answer, which includes some of my opinions (as requested by the question)
I am happy to buy ebooks with watermarks such as 'prepared for Julian Harty' as I'm able to easily copy the file onto the various computers and devices I use from time to time over the years. (I have ebooks first obtained over a decade ago, my computers from those days tend to have died.)
Having a watermark is a low-barrier for people determined to share / copy material regardless of copyright, however for me it's part of an honour-based system where I've legitimately obtained material where I shouldn't share it.
I tend not to buy hard-DRM'd ebooks; the one exception being books from Amazon's Kindle store. The Kindle ecosystem is compelling and one I've gotten used to. One key benefit is global 3G access to content at no extra charge from my 3G Kindles. I'm willing to tolerate DRM on the Kindles, and haven't had problems reading my Kindle content on lots of devices over the years I've been using them.
So for any publishers who read this answer, please realize you're less likely for me to buy from you. I've not bought tens of technical books when I discovered the site uses DRM. Instead I'll find similar books from stores that don't use hard DRM. I want to be able to easily migrate stuff I buy from device to device over the years.