We use SIF and have been using it for 5 years.  It works quite well but you need to make sure the other vendor understands how it works and has someone that is knowledgeable.  We had it for our school website software School Fusion.  The first couple years they had an expert on staff.  He left and so did their support.  We had a lot of trouble with them but finally got it worked out.  
We use SIF for our Destiny Library system, Renaissance Place, & School Fusion and we are starting to look at using in for Active Directory.
We have a District Zone and that seems to work the best for us instead of having each school have one.  Less of a headache.  If you have any questions, feel free to contact me at 402-563-7000 ext 1503 or email stevensons@discoverers.org

We use SIF in Cozad, and have been for the past 8 years or so.  We used to integrate with Destiny but have since moved to their cloud services, which does not work with SIF.  We have SchoolFusion but have never used SIF for that.  The main use for us is Active directory integration.  That is really a great thing for us, a new student is enrolled in PowerSchool and they automatically get an AD account.  That's the only thing we are currently using SIF for, but it is worth it.

A nice thing about PowerSchool is that Pearson also now owns what used to be DataSolutions and they provide the SIF support along with PowerSchool.  I have been able to have them help me get everything set up and working, and they know both systems.  The AD integration has saved us a ton of time and makes EOY much easier, all the student accounts get generated automatically.