Must say that the "blessed()" method is in and of itself worth the use of the entire package... and the package is *fast*. I don't feel uncomfortable at all in making use of it in my other code. It's very lightweight and the methods (such as blessed()) are a huge fastball of usefulness.
The functions offered by Scalar-List-Utils are indispensable. It's hard to believe that some of them weren't in the core to begin with. Scalar::Util is one of the modules I used most often when writing routines that need to validate data or behave polymorphically. Learn and love these modules.
Scalar::Util and List::Util are some of the most useful and most depended on modules in CPAN. They help to increase the clarity of many expressions, especially when you know what exactly what you want to do, and Perl's DWIM'ness starts to get in the way a bit.
The annoyance with it not having any/all functions has gone now we have List::MoreUtils.
The one remaining annoyance with this module, is that as new functions have been added to Scalar::Util it is not always clear WHICH functions are available in what versions and on what platforms. For example, on some platforms weakrefs are not available, even though that version is supposed to support them.
As a result you often need to write unit tests to double check that Scalar::Util has the functions you need, especially as I mentioned for things like weaken.