I've recently upgraded from a late 2008 Core 2 Duo 2.4 GHz Macbook pro (with aftermarket samsung 830 SSD) to a late 2013 Macbook Retina 13 and the differences that have been **material** to my day to day use are:
1. Much faster wifi. I upgraded my old Airport Extreme with a netgear AC router and the read and write speed to my Synology NAS literally shot up from 10MB/sec to 90MB/sec (yes that is megabytes per second).
2. Retina screen. The quality, both in terms of colour accuracy and resolution is absolutely mind-blowing, while being able to display effectively the same resolution without any eyestrain (1440x900)
3. Processor speed (went with the i5 base model). I don't do much taxing work, mostly some lightroom raw editing, general browsing and web publishing so I did not notice too much of a difference, but the lightroom tasks complete much faster. However the old machine after upgrading to the SSD is pretty close to real performance feel.
4. AES encryption natively supported in the CPU. I always have my SSD encrypted as it contains work data and with the old C2D model, the CPU was spiking to 50%+ load when doing write intensive tasks. The rMBP on the other hand never goes over 5-10%.
5. Battery life. The old MBP could barely get over 2.5hours of light use. The rMBP can get quite easily with my workload 9h+ of continuous use and as I only use it outside my day job, it can go literally for 3-4 days without charging. Same for heat. The MBP would get quite warm just browsing the internet, while the rMBP is completely cool to the touch.
Surely my workload is not that intensive and I could get away with carrying on using the 2008 MBP but the above reasons (exceptional screen, super quick WiFi, battery life, heat and slightly overall improved perceived performance) made the upgrade worthwhile. Surely from a technical perspective its at least 5-10 times faster in terms processor speed and triple in terms of disk speed but the machine does not feel five times faster by any means, at least for my workload.
However due to this oversupply of processing speed I don't see upgrading this machine for at least 5+ years. That is at least if my evening work continues to be quite processor-light.