We have had a alternating block schedule for MS and HS. Middle school had electives every day the first year, but after that they were in the block. They take 7 classes and have 3 on one day, 4 on the next. The 8th block is like a study hall period where they are assigned to a homeroom, but can work on homework or projects, or meet with teachers, etc. Occasionally they have all-grade programming for a portion of that period. Most special-ed kids have access to their resource teacher during some or all of this time so they don't have to miss class time to receive services.
The teachers here are used to it and have no problem filling the time. First year language and band are the only ones that prefer shorter periods, but that is why 6th grade has split block for those where they have those two classes every day, but the rest are every other day (it works). Fewer transitions can be very good for some kids and less time is wasted on administrative tasks like attendance or changing for gym. We found it very helpful for our ADHD one that homework was never due the very next day, so if he had questions or an off day, we had some breathing room to reach out to the teacher. I also liked that we could reliably schedule appointments during either the study hall or a class that was not a big deal to miss because we had a 90 minute window.
Mama to three boys ('03, '05, '07)