Crazy good at low levels, and they stay solid in the mid levels if you have the ability to spam them. Still OK beyond level 10 or so, assuming you can summon them in groups, but sort of mediocre then compared to what else you can do.
I played an early-entry Archivist/Cloistered Cleric -> Master of Shrouds a few years back that pretty much ruled the school between Shadow summons spamming and Animate Dead shenanigans, right up through when the campaign ended (level 13). By those latter levels, the Animate Dead was turning into a much bigger headache for my DM than the Shadow summoning, but my group still managed to blow through a lot of encounters by having the wizard lock the biggest opponent down with Solid Fog or Evard's Black Tentacles, and having me surround them with Shadows and drain them to uselessness.