They're good options for folks looking to upgrade from a P&S, but who want a more compact size and simplified interface. Used by a knowledgeable photographer, they are nearly as capable, or equally capable, as DSLRs in many regards, but would require more menu diving to get at enthusiast level controls - for a beginner or amateur shooter, the basic shooting style and technique is pretty much like that of a compact camera - good auto and scene modes, big LCD to frame with, and not much more than a shutter button to deal with.
The most common options for these types of cameras (system cameras, EVILs, MILs...they go by many names) are the Olympus Pen line, Sony NEX line, Panasonic G line, and Samsung NX line. The Olympus and Panasonic models share the same lens mount and sensors, a slightly-smaller-than-DSLR-sized micro 4/3 sensor, and have a decent collection of dedicated lenses for the mount. They aren't quite as strong in lower light situations as a DSLR, but far better than a P&S camera. The Samsung model uses a DSLR-sized APS-C sensor, but is a relatively smaller player with small lens selection. Sony's NEX line also uses a DSLR-sized APS-C sensor, and will perform at the highest ISO levels as well as DSLRs do...they have a fairly small dedicated lens selection currently as they are a fairly new line. The Sony is the smallest, followed by the Olympus Pen models. The Olympus/Panasonic models and the Sony NEX models can be fitted with various adapters to use nearly any lens from any mount ever made, though manually focused only - which has made them a popular second choice for some DSLR enthusiasts who have old lens collections and want to take advantage of a good, modern sensor.
