The only thing that has put me off Fastmail very recently, and indeed I moved to another provider (Runbox), is the loss of their Family plans.
Requiring three accounts now for instance would cost $150 a year minimum for the size I would have needed and unfortunately because I needed to rearrange my old grandfathered plan to re-organise accounts I'd have lost my previous plan.
Basically, Fastmail is good (very good), and you can still do admin type stuff by creating your master account first and creating your 'users' under it, but do look around if you are price conscious.
NOTE - I know you can't please everyone and that doesn't detract from the fact Fastmail was extremely stable, fast and had excellent customer support so don't take this as a 'Fastmail is bad' type post, because it isn't. Just look around and make sure you're getting the rid price/value for your needs.