The only straight up bad experience I've had with fitting into a seat was on a long haul from San Francisco to London in premium economy seating on a British Airways flight. I was only about 285 at 5'6", but the sides of those seats are solid panels instead of the armrests in economy. I got a fairly massive bruise on my thigh from trying to stuff myself into that seat!
The other issue was that the tray comes out of those armrests, too. My weight is predominantly in my belly, butt, and thighs, and this made using the tray impossible. However, I was able to fit into the seatbelt without an extender.
I'm waffling about taking a trip during my break from postgrad studies, as I'm over 300lbs now and I bit nervous after that last flight! I think I'm going to just bite the bullet, though, since the point of getting fat was not to stop traveling! With that, I'll report back if I learn anything of note on this next flight