Our company switched from a custom cms to SilverStripe (SS) last year. Active Record made the migration take much longer than anticipated because of all the building. It also makes data sharing harder. So be wary if you have A LOT of sites like we do that share data.
Other than that...actually developing a site/CMS in SilverStripe is an OO dream come true. No more coding some app feature and then coding some admin and then bug fixing them both and having one work and not the other. I recommend SS heartily for developers looking for a mature active record/php solution that really has some thought behind it. The admin is really cool.
Bonus: Most of the time I am trying to do something and I am like WTF...I just search or browse the code and there's already a function to do exactly what I wanted.
Support: no problems here. There's a forum which is answered when intelligent questions are asked and not already answered. This can only improve in time with more people getting on board and Ingo's SS book getting translated into English.
