The problem with this is not the way he did it - existing immigration laws have considerable latitude for the executive branch written in. The idea that this is illegal or unconstitutional is going to have a tough time getting through the courts.
The problem with this action is that it's only a temporary patch on the real immigration issues. Maybe it's the best possible for the next couple of years but the real problem is the politics that is preventing real reform. In particular a system that financial rewards the people taking the most extreme and intransigent positions more than moderates.