Mirrorless interchangeable cameras like the Olympus Pen and Sony NEX can pretty much equal DSLRs for image quality, given good lenses and a good photographer - so if image quality is the main goal or need, they are a worthy option instead of a DSLR. There are still many types of shooting, and occasionally some types of subjects/photography that a DSLR can do and a mirrorless interchangeable camera cannot - or would struggle mightily to try. So there is still the basic fact that a mirrorless camera cannot replace a DSLR for people who need a DSLR. However, a vast majority of camera users, and even DSLR owners, don't actually need a DSLR. Many people who buy DSLRs never come close to using the DSLR's full abilities, or the specific functions and abilities that make them superior to mirrorless cameras - namely focus speed, burst speed, and tracking focus. Those needing to shoot action, sports, or moving wildlife, or shoot frequently in low light of moving subjects or briefly still subjects, will find a mirrorless camera a poorer tool for the job, and require both skill and luck to get the shots they need, if at all. But those who shoot normal travel, scenic, architecture, portrait, vacation, indoor, etc scenery will most likely never tax a DSLR, and a large-sensor mirrorless interchangeable lens camera can either come close or equal a DSLR's images.
I've not shot with one of the Olympus or Panasonic micro4:3 sensor cameras, but I know in daylight they can match the output of an APS-C sensor DSLR...in low light, they might fall slightly behind at the highest ISOs, but few really venture into those territories. I bought a Sony NEX3 myself a few months ago as a second body to my DSLR, and the NEX with its APS-C sensor is identical across the board with a DSLR for image quality in any lighting situation, including superb high ISO. SO I know what mirrorless interchangeable cameras can do, and I love mine. I still do need a DSLR myself, because I do shoot those types of photography that only a DSLR can do...but if I'm wandering casually around Disney or on vacation and want to travel lighter, I have full confidence I can grab my NEX and get the same quality from it as my DSLR (in fact, I did that just yesterday through Hollywood studios, shooting until almost 11pm with only my NEX with me!).