The K2 is widely regarded as one of the highest-performance rigs in amateur radio. Because it is a kit, it offers challenges above and beyond those of off-the-shelf rigs. However, it is not difficult to assemble a K2. Elecraft has written extremely detailed assembly instructions, provides ongoing support by way of email and an email list/reflector, and the design puts nearly all of the parts on one of several PC boards which plug together directly, with few wires used.
The basic K2 design puts out 15 watts on CW, but its capabilities can be extended with optional add-on boards. These offer SSB, DSP, internal battery power, an automatic antenna tuner, and other features.
Refer to the Elecraft K2 web site for more details.