I'm a big fan of Royal Thorns, at least in the early/mid-game before the attacks start to outpace their HP.
The only problem is that they do seem to throw the weaknesses of the combat AI into sharp relief. Most of the time, enemies are far too eager to attack the summoned thorns. (You can hear the cry in the greedy little AI's brain: "Ooh, if I attack that stack there will be casualties! I love casualties!!") And then at other times, archers will choose to hit the Royal Thorns for 40 damage instead of shooting a stack where they might actually kill something. Either way, it makes me feel a bit sorry for the AI.
But yes, they're a fun unit to use.