I noticed a few weeks ago that the Sigil of Celerity has about the same usefulness as an open upgrade slot. It stacks up fine up to 24 stacks and after that its stuck there. The effect doesnt trigger and doesnt reset either.
Today I wanted to try the Demon Summoning sigil on my MM and the same problem. Goes up to 24 stacks and thats it.
The intended effects of those sigils are questionable enough (5seconds of quickness AFTER the enemy is dead \o/ ), but they are totally useless like this.