Extremely valuable for so many reasons.
Firstly it introduced me to the 9-5 world. Takes a bit of getting used to compared to university (especially when you have a long commute!), but it's not so bad.
Secondly, it taught taught me a lot about the industry I want to enter - not just about how it works but also the different roles available, progression between them and which ones would best suit myself. The knowledge I gained helped a lot when it came to subsequent interviews.
Thirdly, it helped my CV stand out from the pack when applying to graduate jobs. I'm not any better in any way than some of my coursemates who didn't have any internships or relevant experience, but I was more successful in getting interviews than them. Quite a lot of the companies I applied to weren't interested in interviewing candidates who didn't have an internship, year in industry or pre-university work experience.
Lastly, it helped me a lot financially. I was making more than I would have had I worked in a typical student job over the summer.
Overall I would definitely recommend that people take internships if they can get them, and that they take a proactive approach to ensuring they acquire one by not only doing the usual method of applying to them online when advertised, but also calling and emailing companies that maybe don't advertise them to see if you can get some work experience with them anyway.