We've now been using ExpressionEngine for many years here at A Digital. We took the decision early on to specilise in this CMS and make it part of our core offering. We've built a few sites on version 1 of the CMS but the bulk of our work was done during version 2 of the platform. We have a select few sites on the later versions but ultimately we've started to move away from this platform now.
Whilst we may have moved our core CMS offering away from ExpressionEngine and towards Craft, we still have a large knowledge pool and wouldn't be against working with the CMS still. We learnt all of the good, the bad, and the ugly parts of this CMS during our extensive usage of it and have developed many plugins for ExpressionEngine.
Knowing a system inside out is very good because it means that we can very quickly pinpoint any optimisations that need to take place, how it should be set up, and any pitfalls that it may have.
Over the years we've integrated ExpressionEngine into working with EPOS tills, external databases, booking systems, ticketing systems, and enterprise management systems. We've also worked extensively with both CartThrob and Expresso Store to provide eCommerce functionality to the sites that we've built.