Please read Fentonizer review, it is just as good in 2021 as it was in 2015. I love it.
Here, I'm updating the review because it has been more than 5 years since the last one.
Clot is the card to stop fast advance deck. Note that it will not do a thing for Haas-Bioroid: Precision Design deck which make use of Seamless Launch.
But against corp which use tools like Remastered Edition, Biotic Labor, Focus Group, Trick of Light, Psychographics and SanSan City Grid, the "Clot Threat" is a show stopper.
Getting rid of Clot might be as simple as doing a purge, but a purge rob the corp of a turn (outside of using Cyberdex Virus Suite or Reverse Infection). Which, in itself, is already a heavy price ( are precious).
But when you combine this with Simulchip, you can bring it on the board as soon as you see the corp trying to score an agenda in fast advance mode. Which is why the corp should always ask the runner for an action when there is Simulchip on the board, before putting the last advancement.
Of note, you can also use Gachapon to do the same thing as Simulchip, but you'll be less sure of the result.
Clot gives you a fighting chance against fast advance corps. Otherwise, you might lose by turn 7, which is often too soon to get your rig working.
Because of the Simulchip combo, Clot is most often used in Shaper deck, which are usually slower to build their rig, thus needing that fighting chance against fast corp decks.
Great card, often essential in many competition. Evocative art and quote. Well designed.