Didn't like the first, didn't like the second.
A complete engine overhaul make this one a true eye-candy. Also, multiplayer support has been revamped and improved, which is also good. But it's otherwise not very different from the original.
The same as in I: an unintuitive control system and unclever level design. The sound effects have been improved but are still sub-par. The music in Descent II is not nearly as good as in the first, and gameplay is just plain boring.
The Bottom Line
If you liked I you'll like II, though it's not really different.