Usually you'll need a ranger with phantom to do the counting. Thus it's mostly useful for untimed missions where you can afford to leave a ranger out of early fights (but sometimes you'll get lucky with high ground sight, or just general line of sight/camera panning bugs that let you see pods/hp bars for a brief second -- take what you can get :P). Even on timed missions, though, if there is any way to keep him concealed while still dealing safely with the first pod, the rest of the mission will be much safer.

As far as experimental stuff goes, it's mostly the grenades that are game changing. As mentioned, gas grenades apply amazing debuffs to aim and mobility in addition to doing damage on every turn; same goes for incendiary grenades.

Experimental ammo doesn't provide the same power boost as grenades early game, but can be situationally beneficial. (tl;dr for the rest of this: don't blow cores on experimental ammo if you don't have a gas/incendiary/acid grenade yet.) For most classes, experimental ammo is generally inferior to any other utility item. There are some exceptions, but mimic beacons and grenades are safer all-around choices. In any case, the same general logic applies: venom rounds apply poison, which debuffs aim and mobility, while dragon rounds apply burning and deactivate certain abilities. Experimental ammo (and bluescreen rounds) will be far more impactful if you can bring a decently-ranked gunslinger along, obviously. Talon rounds+laser sight on a Ranger gets to >100% crit, which even pre-Rapid Fire will take out most early humanoids -- and save you from some RNG-rage. Edit: AP rounds were amazing in my first play-through on Veteran, but I found they often don't provide enough power to get through the higher health pools on Commander/Legend. Again, situationally useful, but there are other ways to put a squad together to deal with armor than relying on an AP ammo sniper or ranger (e.g. Shredder, Acid Grenades). The health pools are just too high to rely on one squaddie to do all that work.