I had two 17" MacBook Pro's (2006 to 2009 then 2009 to 2013) and I really really wanted Apple to make another 17" notebook. A retina 17" with a 4K display would have been an instant purchase for me.
It didn't happen so I decided to wait a year (June 2012 when the rMBP's first came out) to 2013 for Haswell. I held out because I was hopeful that maybe a 17" with 4K would come and it did not obviously.
So I bought a 15" maxed out Haswell, the one in my signature and I think I can give a good perspective on this having owned two 17" (One classic and one Unibody) and now the 15" rMBP.
Obviously the display on the 15" is incredibly sharp, much sharper than the 1920x1200 display on the 17" it is easily noticeable in only certain conditions however. I run my rMBP at a scaled 1920x1200 resolution which means it renders the desktop at 3840x2400 and then scales that down to 2880x1800 to fit the display panel. It looks much sharper than the 17" does but it's not really that noticeable in normal use only if you're up close like the sort of closeness required on a plane in economy class.
The weight difference is very very noticeable. I can easily without any effort or strain hold my notebook by its side with one hand, like you would with an iPad. I wouldn't recommend doing this for an hour of course but moving the notebook around is much easier. It is also drastically thinner, the display is a lot thinner not only the base.
The keyboard on the rMBP doesn't feel as good as the 17" keyboard in my opinion. The keys feel cheaper and don't go down as much as they do on the 17" - I think this is something you could get used to in time (I already have) but the 17" keyboard is just better.
The 802.11ac wireless on the new Retinas is very very good. Incredibly fast with great range. The PCIe SSD is also insanely fast. In some tests I did I was almost reaching 1.1GB/s writes. The best SSD in a 17" is going to be about 550MB/s write at best which is not slow by any stretch and you always have the option of putting a second SSD in the 17" and using RAID0 which would reach 1.0GB/s writes if you used the fastest SSD's available.
The trackpad feels the same on both notebooks and it's the same size. It's responsive as ever.
Having HDMI out on the 15" is convenient but if you never hook up to a TV, not needed. The 15" can also drive two displays over Thunderbolt simultaneously like two 27" or 30" panels each at 2560x1440 or 2560x1600. The 17" with its single Thunderbolt port can only drive a single 27" or 30" display and it doesn't have native HDMI you need to use an adapter.
The lack of an ethernet port on the 15" may affect you or it may not. Personally for me I picked up a Thunderbolt to Ethernet adapter and although I will seldom use it, it does work perfectly with no noticeable CPU overhead and gigabit speeds.
Obviously for me I went with the 15" because I found it more compelling than a 2011 17" - Had Apple released a 2013 17" with Retina display I would have jumped at owning one but I think the 15" is a respectable replacement. Be forewarned about what I mentioned though, the keyboard on the 15" isn't as good, it doesn't have native ethernet and you're limited with the storage being proprietary and the RAM being soldered. The 17" is still a powerhouse with great performance and I think in a lot of ways the 15" rMBP tried to change too much too quickly making the 17" a very attractive proposition.