Connectivity is a deal-breaker
The biggest problem with these machines is the lack of connectivity. We've been buying a few for the office as they're an attractive alternative to the full MacBook Pro for those who don't want the weight, but the connectivity is abysmal. You need a second display as the screen is too small and too low-res to work from for the entire day, but you can't have that plugged in at the same time as a Gigabit Ethernet link as they both need the Thunderbolt port.
The only alternatives are keeping them on 100MBit (too slow for our needs), a £300+ third-party dock (with all the attendant compatibility concerns that brings), or Apple's own Thunderbolt Display which has a network port on the back.
I had a stand-up argument in the Apple store when I was told that third option was my only option as to have to spend another £900 to make an already very expensive laptop usable is unforgivable.