Depends. Nuclear thermal rockets are superior. 800 to 900 "seconds" of specific impulse. Superior "performance".
But look at the thrust-to-weight ratio:
Nerva/Kiwi/Phoebus never got much above 7.5 thrust-to-weight. The SSME is ~70 in thrust-to-weight. Some of the expendible engines designed in the 1960s were close to 100 thrust-to-weight.
Even the "Timberwind" nuclear engine was only about 10:1 thrust-to-weight.
That's why nuclear thermal rockets are not good for boosters; they are simply too heavy--not to mention the danger of launching an operating reactor from the ground. The abort scenarios alone are horrifying.
But for in-space applications, where raw Isp matters more and thrust-to-weight matters less, they would be great. You can launch the reactor "cold" and with the reflectors retracted. In such a condition you could go up and hug it without fear of radiation. Once you turn it on, it is "hot" and therefore dangerous. Indeed, disposal of the engine once you reach your destination or return home is a big issue. The "obvious" solution--dropping it into the Sun-- is too costly in terms of Delta-Vee, reaction mass, and $$$.
Plus, the eco freaks would riot, protesting our wanton pollution of the Sun.
--Bors