I've been commuting on southern from Oxted to Victoria for about 3.5 years now. It has improved significantly, resolving the industrial action obviously made the biggest difference but they seem to have fine tuned the time table. We rarely get stuck at the South Croydon junction waiting for a path to the main line any more although we still get plenty of time stationary admiring Selhurst depot waiting to cross that junction! While there is a lot of low level lateness (5 minutes here and there) it is very rare i claim delay repay these days. Problems that do occur are usually due to things outside GTR's control, whereas there used to be lots of stock shortages and staff shortages and "planning errors"! The 377s are also pretty good and better maintained than they used to be, it is a pretty comfortable commute as long as I can avoid the dreaded 3+2 seats and they are rarely short formed.
Having said all that, the Uckfield service through Oxted seems a lot more fragile than the Victoria service and I think may have gotten worse. They don't seem to have enough stock or recovery time and they are a lot more prone to be cancelled or short formed. This causes a lot of problems in the evening as it is one train per hour apart from the main peak. There used to be half hourly shuttles at least until a certain time in the evening but these were scrapped to lengthen other services. I often see poor Uckfield passengers settling in at Oxted for a long wait or boarding buses.
Thameslink has been a mixed bag. Our stopping services to London Bridge were handed over to Thameslink in the big timetable disaster. It was a complete shambles then, now it has improved a lot but they do seem to be cancelled or significantly delayed much more often than Southern. However, when they are working OK the connections they offer are really useful and seem to be appreciated judging by the big proportion of people who stay on the train past LBG towards STP.