Let me start by saying I do believe the zodiac system overall casts very broad descriptions so everyone finds some truth in how their personality aligns with their sign. That being said, I am a Gemini and I more closely align with the Gemini attributes than any other sign (or maybe that is because I've been conditioned to find the similarities). I would not say Geminis have "split personalities" but it can sometimes feel that way because we can be very indecisive (usually caused by boredom) and that can also cause mild mood changes and frustration.
Supposedly, Geminis are most compatible with other Geminis and I find that to be pretty accurate. My most compatible relationships have been with other Geminis. It's easy to stay entertained when both people get bored easily (that means two Geminis together will always want to do new things, sexual and otherwise
). Make no mistake though, this doesn't mean Geminis are more prone to cheating or anything, you just have to be very refreshing otherwise a Gemini won't even waste their time with you to begin with.
And you have to put up how drastically Geminis change. For me for example, my friends describe me as "going from 0 to 100", if I'm on a "healthy kick", I'll be eating healthy and dieting for weeks and then I have my "lazy kick" where I literally don't want to leave the bed and want to eat ho-ho's for dinner. My boyfriend understands this and tries to help me find balance but ultimately it does sometimes feel like two different minds are controlling me lol. So things like that you have to understand with a Gemini. As long as their "split personalities" are not detrimental to the relationship (ie: one is not abusive or something).
Overall, Geminis are fun and will keep you entertained but they can get bored very easily.