Diamond seem to be aiming at the FL250 market (just above most weather) which is quite different to the FL350+ market which Eclipse etc are going for.
There is IMHO a nice gap in the FL250 market, for mostly 1 or 2 person missions, because the only present contenders are turbocharged pressurised twins or turboprops. The former are mostly very old (and not "nice" unless you comprehensively refurbish one) and the latter are very expensive if bought new.
I am not sure if the D-jet is under 2000kg but if so that is a dramatic operating cost saving for European IFR, a couple of hundred quid on a long leg, compared with a > 2000kg piston twin. The only piston twin I know of which is < 2000kg is the Seneca witht he STC mod and that is not pressurised so FL250 is not a realistic habit to get into, especially with passengers.