Good, unquestionably. Provides the energy necessary to achieve warp speeds and power, and also gives Scotty and Geordi something to do when they start to fail. [Reply]
I wish more could be said for the Lang Fusion Cycle Engines used on the Promellian Battle Cruiser. Though they are crude and underpowered by 24th Century standards, they worked well for generations. I don't believe they utilized Dilithium either.
Of course the only surviving Promellian Battle Cruiser, Cleponji, had to be destroyed by the Enterprise D due to the Aceton Assimilators planted by the Menthars.