Existing reviews have outlined briefly the conditions under which Crystalline Elder Sign is good and bad perfectly well: it is best when played by a solo investigator making at least some use of all their stats, and worse the more investigators you play with, or when you mostly just use willpower and/or intellect and St. Hubert's Key would suffice. I'd like to delve just a little deeper into why this is the case, and why Crystalline Elder Sign is good when it is, because 3xp is a hefty investment and you should get every bit of value you can out of that.
Removing a +1 or token from the chaos bag is bad because it removes one token that would have been a success for you on skill tests where you were at least even with the skill test. By getting +1 to all your stats, however, you make new tokens that would have otherwise been fails into successes. For instance, after sealing the +1 on Hard Standalone Threads of Fate, with three or more doom in play, you improve your success on a skill test that you would have been 1 over from 6/15 to 9/14 on a skill test you're now 2 over, thanks to the +1 stat boost. Your odds of success in this case go from 40% to just over 64%. This is pretty good! There will be similar cases in every scenario, and this improvement will be across all your stats meaning it is for every single skill test you take after you play Crystalline Elder Sign.
The downside is that sealing the +1 or hurts your teammates. Now, removing one success token won't have a huge impact on any test where your teammate was already at a reasonable advantage, but that effect adds up across multiple skill tests, so you better be pretty justified in how many benefits you reap from your boosted stats. As mentioned, you really have to use your combat and agility at least sometimes to justify Crystalline Elder Sign over St. Hubert's Key, and this is a problem for most Mystics who want to turn their problems into ones they can solve with their superior willpower by the use of spells. One standout exception to this rule is Jim Culver who can leverage his average combat with Enchanted Blade and its upgrade along with his -modifying ability to make the chaos bag more amenable to making marginal skill tests, including incidental intellect and agility tests from investigating and drawing treacheries.
There are other cases where Crystalline Elder Sign contributes even more. Sealing a +1 or slightly increases the odds of drawing the special tokens that many Mystics cards look for. This is particularly strong for Shards of the Void, Song of the Dead, Sixth Sense, and Wither, all of which gain powerful bonuses on revealing 0s, s, and and . On the other hand this is a downside for the Mystics classics Shrivelling and Rite of Seeking. One fun corner case where Crystalline Elder Sign's sealing is good: removing the actually benefits Daisy Walker should her weakness The Necronomicon be in play. Shame she's just short of it being in her card pool!
Other reviews have already stated the conclusion of mine: play Crystalline Elder Sign in lower player counts and when you are actually regularly testing combat and agility. Ignore it when St. Hubert's Key or Holy Rosary will suffice, which will be most of the time.