I've done a lot of research into exchanges and I would say Gemini is one of the top ones out there at the moment, provided you understand the technical background of the exchange and are using the exchange for legitimate purposes.
One of the biggest advantages with Gemini is that the vast majority of coins are kept in cold storage, i.e. if Gemini were somehow hacked they would lose only a small portion of all coins on the exchange. Of course you should be transferring everything into a cold storage wallet of your own but Gemini's cold storage is a nice safeguard for those who n00bz who don't know better; sadly, there are a LOT of n00bz out there in cryptoland these days.
Other major advantages include the low fees, verification process for fund transfer and allowing you to trade with bank-transferred funds which haven't fully cleared (would obviously be reversed if funds didn't clear). Two factor authentication and especially connection to the Authy app is a nice bonus but not too unique for exchanges these days.
The only major cons I can see to Gemini is a lack of analytical graphs and chart options. I'm an extremely analytical person and love having heavily detailed graphing information. I have to go to other sources to obtain detailed charts and related material.
Lots of other users and reviewers have complains about significant downtime but I have only once experienced any issue with Gemini downtime so far. Please realize that they very likely shut down temporarily during extreme demand times, Gemini and most other exchanges only have so much capacity for trading volume; this isn't the Dow or NYSE so don't expect the same out of crypto exchanges.
A copy and peste should be a quote also have the link to the original.
https://www.cryptocompare.com/exchanges/gemini/reviews