I upgraded to FF a few weeks ago, and the day I received my 5D I used my 50 1.8 MK i to shoot some of my friend's birthday dinner.
I was truly blown away by the shallower depth of field, and overall rendering of colors in a low light environment. I used to have an XSi, then a 40D with a 50 1.8 MK ii and I remember really sharp images, but bokeh was not as good as it is on the 5D with the same lens (lenses are same build in every aspect except MK i is a Metal Mount). I mostly shot portraits of course when I had that set up, since it was an 80mm equivalent on those crop bodies.
Overall, I've shot for a year, and upgraded finally now to FF. I'm a university student, so my budget has been low, and gear wise, I've made careful decisions to what I would actually need and would use and learned and made the most out of it.
In general, I've always used one lens for the most part, ever since I picked up the 50 last year. Never went back to the kit lens and just kept on moving forward. Sold off XSi kit, then realized I wanted something wider when I went 40D, so I got the Sigma 30 1.4. Never went back to the 50 when I had it since I loved the 50mm equivalent length. Sigma IQ was just fantastic; also great bokeh, silent focusing, sharp when needed, excellent ergonomics for me, and real time manual focus override.
Atm, I'm preparing to study abroad in Taiwan next semester. I have my 50 MK i right now but I don't know if I should upgrade to the Sigma, or pick up a Canon 35 f/2 for wider angles (particularly for indoors, landscapes, night life since it's fast, etc.).
For me, there's been numerous times where I've felt like I needed a wider angle, but I'm not intuitively feeling whether it would justify picking up a 35 for that reason since the 50 range with the 30 on a 40D fulfilled around 95% of what I have shot so far.
I'm twisted right now, since I haven't shot enough with my 50 on 5D to know if it truly fulfills what I need. I know Sigmas are a hit and miss as well, but I would plan to buy a used one with confirmed sharpness and good AF. But so far, at least in low light, I am still pretty shocked at what I have taken from that one night.
I guess after hearing my story (sorry for the length), would you think it's worth spending 40% more for the Sigma for IQ, Bokeh, and Silence? Or would it be better for me to just pick up a 35 and have a wider angle, but still pretty good IQ due to FF sensor rendering?
Sorry for the length again, but for more detail, I mostly shoot for memories, high quality memories. I do a lot of hiking and nature stuff, and also shoot food and friends. Occasional events for campus orgs that my friends are in and such as well, but in general, I shoot a lot, and edit a lot, probably more than most people who are not doing photography for profit.