I've just spent a most enjoyable long weekend down in Newhaven, and I'm very pleased to say that the train journey, both ways, went without a hitch (and for only £36.50 each way as well).
Every train left on time, and if a couple of them arrived a few minutes later than scheduled, that just meant a bit less time to wait, but not so much as to cause panic. The longest wait I had was half an hour, and most of that was spent on the (stationary) train.
At Brighton I finally remembered that I wanted to be in the middle of the Thameslink train, not at the front. The Thameslink was less crowded in the middle of a weekday than it is at weekends.
No one sat next to me on any train, either way. On the way back there were three separate reservations (St Pancras to Leicester, Leicester to Derby, and Derby to Sheffield) for the window seat and not one of them was used.
It was slightly unnerving that the Seaford train (313208) left Brighton from Platform 7, but the correct order of the universe was restored when 313210 disgorged me onto Platform 8 on the way back.
I took a few 313 interior photos, because who knows how much longer they'll be around?
When I got back to Sheffield, I got the tram from the station. I waited five minutes and it cost me £2, and saved me a long and hilly walk (although there's still a hill at the end).
There is just one, very tiny, irritant. Why is it that despite it being a perfectly valid route (and indeed, one suggested by and pre-booked via the National Rail website) my tickets never work in the Thameslink barriers at St Pancras, in either direction?