>What I'm wondering is how you've experienced using SilverStripe over time?

I love it ! Sure there are still some important issues to tackle (e.g. performance on large sites / URL hierarchy) , but all in all it is by far the most elegant Open source platform I know.

>Are upgrades an issue? Does it take a lot of work when you
>have multiple sites to do it on, does it break modules, widgets, translations?

Yes, it takes quite a bit of work to upgrade every separate site and you might experience some modules and/or widgets breaking down , but in my experience it mostly concerns fixing details / changing some config settings. It is nowhere near the problems you'll have with Mambo / Joomla and the like.

>Have an upgrade ever suddenly been so drastic that you have to completely redo a site?

Nah. And I don't expect that to happen in the future, since the core code is clean , well kept and based on a good (extensible) architecture.