I have the 1.8 and 1.4
In my experience, the 1.4 is the better lens - just so long as it continues to work for a few more years. At the price though, I won't be heartbroken if it does and by then, who knows? Canon may have the long awaited replacement.
Used it on our tour of the Pacific North-West this summer. Perfect for low light shots with my 6D - got some low-light shots simply not possible with many other combinations (certainly not at the price point!)
Low-Light ISO 10,000 f/1.4http://www.flickr.com/photos/22983840@N08/9725838260/?reuploaded=1
And shots for "pop"? - I'm pleased with this. Just love the contrast and colour straight from the shot without the need to post process:http://www.flickr.com/photos/22983840@N08/9661037319/
So yes, an old lens, but still a worthwhile addition - especially for those without a 50mm