The experience will vary depending on the firms own preparedness and expectations.  Epicor and their top partners are all capable of routinely delivering good service, but they do not always get it perfect either.   Sometimes there is a question of fit between the consultant(s) and the firm.    

There are exceptions, but in-house self-installs do not have a good track record.   

Implementations are largely about knowledge transfer (consultant to client).   As they progress, the client should gain enough knowledge and confidence to drive the efforts.  

We used a vendor team (re-seller) and I'd give them a 3.5/5.