Disciples is a game of attrition. Your armies grow in strength slowly, resources are claimed as your influence increases, and there is very little benefit in fielding a ton of low level troops - a single strong army can trump wave after wave.
At the same time, the game gives a visceral sense of achievement as your parties roll over the lands and harry and choke your opponents. It has that magic sauce that Sid Meier calls the "one more turn" factor. Each level-up is felt strongly as they can completely change your gameplay with that party - a healer might gain the power to heal all, while a leader might be able to add another unit to the group or be able to weild a powerful artifact.
Very well made game, no bugs, excellent balance between the races and troops, a good campaign and lots of replay value make it a worthwhile buy years after release.
I own both this and the sequel and all its expansions and every now and then spend a night playing through a mission even though I've finished both a long time ago.