Caveat, it was a long time ago. Second caveat, I ended up completing with effectively positive money afterward. However, generally yes anyways. Towards your specific questions.
Knowledge you gained of real value?
Yes, mechanical engineering, and most of the skills / knowledge have been used frequently for the last several decades. Job immediately afterward was designing optical instruments, and mechanical design skills were directly applied for several years. Later job was with NASA / MSFC contractor (this kind of stuff [1][2]), and engineer skills use was pretty much all we did. Mechanical design, acoustics, fluid dynamics, testing / experimentation, problem identification and resolution. Actually, not totally fair, we also wrote a lot, and college English classes + graduate research work helped a lot. Surprising number of publications for working in .gov. [3]
[1] https://ntrs.nasa.gov/api/citations/20140016892/downloads/20...
[2] https://araesmojo-eng.github.io/araesmojo-html/project_nasa_...
[3] https://araesmojo-eng.github.io/araesmojo-html/resume_public...
College had a large scale senior design project, and the organizational skills necessary over a year for a significant size team were also helpful in being able to consider a problem with large scope beyond your own personal ability to tackle, work with others to frame the problem, consider a solution, and then implement it. Also involved working with a customer (large chip manufacturer), external vendors, and other teams in the same design project facility. All those skills have remained useful for years.
People often complain that classes like math are not especially useful (or the usefulness is not communicated well). Yet, over several decades, algebra, geometry, trigonometry, calculus, differential equations, continuum mechanics, and numerical methods have all proven useful. Admittedly, engineer. So, kind of a profession biased towards actually using math.
Well prepared to enter your field?
Yes, no real complaints. First job, felt like I made a difference in their products, and was able to achieve something relatively quickly after starting. Skills learned were applicable, and some the design stuff mentioned earlier came up almost immediately because we had a machine shop on site. NASA was a similar situation. Been a little while since studying the material, yet most ended being necessary parts of the job almost every year. Day to day work involved significant use of engineering classes, math classes, writing / English classes, and even a bit of electives stuff like economics (lots of money in rocketry), psych (real mindset / fear / disposition / perception issues in launch or not), history (grappling with a 25 to 30 year old program (Shuttle) and ~70 year old institution), and poli-sci (it's .gov).
Do you feel your degree is more than a guarantee to employers?
Yes, much for the reasons listed above, and many of the skills have simply been valuable in personal life outside of the work environment. Hobbies, friendships, ability to consider types of problems I might not have otherwise, awareness of other fields of education, awareness of the variety of research activities, being able to read difficult material without any immediate reward, proofreading my own and other's writing (had several errors in this post...), self directed research and investigation, finding info with limited clues, and being able to formulate somewhat long form responses to questions like yours.