The difference between a stock GI trigger and a Geissele trigger isn't really apparent until you have one in your hand. The SSA's trigger has a unique hook protruding to the top where it engages the hammer where a stock GI trigger lacks the feature and the disconnector has a unique design where the front portion of it goes through an opening in the aforementioned protruding hook of the trigger body. The hammer has been redesigned as well and the hammer is meant to be lighter to be able to strike faster but the spring on it is a full powered spring as to not risk any light hammer strikes which translates to consistent ignition of the primer and powder.
A very neat tool that the SSA ships with is the slave pin which is a small pin that is the same diameter as the trigger pin but is only long enough to fit flush within the trigger assembly. This is intended to hold the trigger and disconnector captive to one another during installation without the need to fumble and curse the disconnector as you line it up in the lower receiver to pass the trigger pin through. I have done a similar method where I used a punch to hold the trigger and disconnector captive in the lower and follow the trigger pin through but the slave pin is certainly an easier step than fitting a punch through.
As for the installation of the SSA; if you've ever installed an AR-15 trigger than you will quickly realize that the SSA trigger install is no different. The manual is quite clear as seen below and about the only moderately annoying step was the hammer install with the full power spring. The hammer spring felt harder to compress and line up to the lower receiver's hammer pin holes compared to a GI hammer spring and it felt as if there was more resistance. I am saying felt since I do not know for certain if it actually does have more resistance compared to a GI hammer spring.
The installation went quickly and in no time I arrive to the step that I have been looking forward to: trying out the trigger. When slowly pulled you will notice the short take up of the initial two pounds than a wall where the second 2.5 lbs. stage that breaks crisply and as described by Bill Geissele himself, "like a carrot" and the trigger resets and breaks at the same location every time which is really nice to train and shoot with. When the trigger is pulled with speed the trigger breaks quickly like a single stage since the initial take up is short and the two pound take up under stress is negligible. The trigger break being predictably crisp it was a breeze to shoot along with the reset being pleasantly shorter than I expected which produced very quick follow up shots. I am pleased with the trigger's performance so far and look forward to trying the trigger out to distance, which is the final use case criteria I presented to Bill which is why he recommended the SSA over the SD3G stating that the SSA should produce better groups in distance compared to the SD3G. If you are deciding on a new or upgrade trigger, get the SSA as it outperforms the RRA Two Stage Match Trigger easily and at the price point, it simply can not be beat.
Now to decide on a stock for this build....