I'm a beginner (played before but getting back into the game)
They either seem really minor (Vial of Blood ime) or run defining (Eye of Newt was great on Assassin)
Alchemy was a pretty intimidating system to get into, but Trinkets, because they're self contained and seemed to promise a minor effect, were a fun, low resource cost way to experiment.
I usually just make whatever seems fun and discard it if it proves harmful or if I need the inventory space. Upgrading them doesn't seem worth it, but I dump excess energy into them if I have redundant consumables in the Caves.
I love the fact that they can be discarded when needed, so their effects could totally be made riskier imo, but that could make them tedious to manage.
My current run has a +3 Exotic Crystals, which I took because I'm less familiar with exotic pops, and while I didn't like what I was getting, I kept it because I knew I could throw it away if I got low on regular stuff. This led to a fun encounter where I got surrounded, and popped a Purity potion followed by a Corrosion potion. I would never have made a Corrosion pot, but now I have a good reason to thanks to the Trinket :)
Yup. I used all my infusions trying for something good (apparently Annoying is good but I didn't know).
I didn't have any infusions after hitting Lvl 30 and was hoping that it retroactively would turn into a +2