I'm gonna buy it as well... I always get a new phone every two years, and yes, the design hasn't changed a lot, but there are plenty of new features compared to my iPhone 6.
Also, I got the 6 Plus when it was new, brought it back and got an iPhone 6 (because I found the 6 Plus to be too big), but this time I'm definitely going for the Plus again.
Not sure yet, but 1 GB of ram isn't really enough anymore for me, so I don't know if I want to spend another year with this phone.
After two years, I can (you don't have to) bring them back to my provider, when I get a new phone, and three weeks later I get a 'new' refurbished phone.
So, when I get my 7 Plus, I give my iPhone 6 to my provider and get a refurbished one a bit later, that I can sell or give to somebody else. I always give mine away.
You can only do this if your phone isn't damaged too bad, and I always wear cases and screen protectors. My phone has only one scratch in a corner, which they won't care about.
You don't have to, but I do this because I don't care that I don't have my phone for 3 weeks if I have a new one, and I get one with a new battery and that looks pretty new.
I always get a new phone every two years, in the beginning of the year (Early 2011 -> iPhone 4, Early 2013 -> iPhone 5, Early 2015 -> iPhone 6) but I'm gonna change carriers so I'll probably try to get mine in October or November.
It's better to wait a little because you never know. Apple sold iPhone 5's with bad batteries in it when it came out (September 2012) and they changed something with the processor of the iPhone 6 in November 2014 (in Europe at least), so I want to make sure I have a 'good' one.