Ive had a script of gabapentin for a while now to try to help keep my benzo usage low, and ive found the effects to be highly inconsistent. Like 1 out of 5 times it will actually give me the sedation I need. The other times it has little to no effect. And tolerance develops quick. Plus, the bigger the dose gets, the smaller the bioavailability becomes. Once you get much past 1600mg, its almost worthless to take any more because barely any of it will even be absorbed by your body. You have to take somewhere on the range 3-5 grams to actually get any effect if you're already fully tolerant to a 1600mg dose.
And as Jasoncrest mentioned, it does have some strange trippy effects. Id almost classify it as a disassociative effect, with a dash of schizophrenia. It can feel uncomfortable at times. Regardless of these downsides, I still have a special place in my heart for gabapentin. I'm just looking forward to when they have a generic of lyrica available, which won't be for a long time.