I have this. We use the checking account for mortgage and transfers into our Fidelity brokerage account, so not our main checking/debit account. I also have the Fidelity Retirement Rewards Am Ex, which has a decent rate for cash back and/or transfers into an IRA.
I love the customer service of Fidelity, and I love how easy everything is with them. It's also very convenient for me to move money between accounts as needed.
The con would be the lack of branches. There is one about an hour from me and that's about it for this area. Generally I don't need to use the branches, but in a VERY rare occasion there is something you have to do in person. For me the only times that has come up is when I added my H to my accounts, and like, twice where I had checks to deposit that were greater than $10k (which does not happen often to me). That is the max you can deposit through their app.