For general use, even the smaller sensor M4:3 type cameras will rival a DSLR for overall image quality. As you get to specialized use such as extreme low light, the APS-C sensored mirrorless like the NEX are the same as APS-C DSLRs for image quality since they share the very same sensor. The only areas where the mirrorless cameras aren't quite a full match for DSLRs is in the native lens collections of which none come close to DSLRs in size and scope, in focus speed and tracking speed when dealing with fast-moving subjects or low light, and in some of the overall function speed such as continuous frame shooting at full res RAW (buffers tend to be smaller than on DSLRs) and battery life (batteries are definitely smaller than DSLRs).
I personally use a NEX-5N alongside my DSLR. I don't consider it a 'replacement', but a compliment - it acts as a great lightweight second body, and it can take the place of the DSLR when I want to or need to travel lighter or be less conspicuous. Image quality of the NEX-5N matches that of my DSLR assuming lenses and lighting are identical, which it well should since both my DSLR and NEX-5N have the identical 16MP sensor!