I had almost every iPhone to date (I skipped the 6 and opted for the 6 Plus, and skipped the 6S for the 6S Plus) and I am used to the iOS ecosystem.
On the hardware side of things this is the fastest iPhone you can buy, but, it isn't too much faster than the iPhone 6S Plus and if you are a 6S Plus owner I do not recommend upgrading to this iPhone. If you use the iPhone 6 or 6 Plus or prior I do recommend upgrading.
The dual camera is a nifty idea but I tend to never use it and I highly doubt majority of people will use it, the image still gets blurry when using the 2x camera (less than zooming in but still blurry). The "normal or 1x" camera seems to be on par if not a tinch better than the iPhone 6S camera.
The water resistant IP67 certification is a great plus to the iPhone 7 and 7 plus range but I tend to not get my phone wet regardless, if you are the type of person who has liquid damaged a phone in the past than this is great if not I doubt you'll even notice and treat this iPhone as if it were not water resistant.
The removal of the headphone jack (as controversial as this is) is not that big of a deal to me. I do listen to music a lot on my phone but the included adapter gives you a headphone port that you can use and I have moved on to bluetooth headphones. The adapter provides you with the same audio quality as you would get on a iPhone 6S Plus via the 3.5mm headphone jack.
To address why I do not think this is a big deal. Apple is still an innovative company. When you look back about 8-9 years Apple were the first ones to make computer without cd drives and removal batteries, then they made laptops that you couldn't really upgrade and now days almost every computer on the market is like that and every company and followed Apples lead. I believe the headphone port is another example of this.
The home button is now a piece of glass that does not move.... As weird as this sounds Apple uses the force touch/haptic feedback (basically a vibration feedback) to give you the sense of you pressing something when you're really not and I can for one say that this works 100% perfectly fine. I can not even tell that I am not pressing something down and find this truly amazing. The thought of this may seem weird to you at first but it is honestly great and I love having this home button.
Now on to the dual speakers. The iPhone 7 Plus uses the ear piece (where you commonly use to listen to someone while on a phone call) as a secondary speaker. The iPhone does seem a bit louder but this isn't really a big deal, I'm sure if you're fine with the speakers you will barely notice any change besides the difference in volume.
The battery life on the iPhone is the same as it always has been in the past, depending on the type of user you are you might find the iPhone has great battery life or horrible battery life. I use my phone constantly and find that I can get from 7AM to 3PM before needing a quick recharge.
Lastly the screen on the iPhone seems sharper than previous generations. Apple has touted they have changed the color gamut and how they've allowed the screen to be brighter and produce more colors and this is true, I have notice a difference between the iPhone 6S Plus and the iPhone 7 Plus.
Like I mentioned before if you have a iPhone SE, iPhone 6S or iPhone 6S Plus don't upgrade if you have something older than this would be a great upgrade.