The extended slide release is a good product, and I don't use it to release the slide. The way Gaston Glock designed the pistol to release the slide is by reaching your left hand palm down, across the top of the slide, pinching the slide between the heel of your hand and the first pads of your four fingers, draw the slide back, and release.
I use the lever to lock the slide back. I thumb it up. It is MUCH easier for me to thumb up the larger extended lever with the reaised triangle on it. Really helps when clearing the weapn, doing a malfunction drill, etc.
The accessory I do not like, unless it is a range only gun, is the extended magazine release. Stock is fine, and I know more than one person who has popped a mag by sitting down with the GLOCK either mexican or in an IWB that either did not cover the release or was not rigid enough. I took mine off my G35.
My "got to have" list is 3.5 connector, extended slide release, Wolff magazine springs, M3 tac light, and Express (AO, Ashley, whatever) night sights, kydex paddle holster, minimum 3 high caps/gun, and a lot of Georgia-Arms ammo.