They were very flexible during our first implementation as well as our second one! As an organization that sells high-end products we knew what features would be needed from day 1 but they listened and accommodated us when we had questions or concerns about how certain systems worked together - it was wonderful customer service at its finest!! I can't think of anything bad so far aside maybe if there's something not addressed yet? Ask them all your questions up front.
It will save you time later when issues do arise and solving problems is easier because they've already thought through those potential roadblocks for their clients before going live. Our Account Managers are able to close more deals than any manager could possibly handle by themselves alone (we're talking thousands per month). By bringing additional resources onboard this has allowed me and my team better efficiency which translates directly to revenue growth.