SEN does seem to be successful
No reason for it not to be. Allowing for a settling down period, nit picking about whether its London or not (apparently being in West Sussex or Bedfordshire is no bar to being an official 'London' airport) and having good links to the city, it seems to be proving its value. I know a few people (non aviation buffs) who have used it already and, despite some comments about 'ooh, innit small though? is it a proper airport?'
, all of these people now favour it and are recommending it. I'm sure we'll see some tweaking of routes, some new operators arriving and some probably disappearing - all of this is part of the genesis of a new or re-generated airport. Look at LCY for the first few years - a myriad of routes and operators came and went until it settled down. There is a place for all six of the London airports, between them they serve a vast and diverse population that needs to travel quickly all over the UK, Ireland, Europe and the world.
The only negative comments I'm picking up at the moment are about lack of early morning/late night connections from London and the fact that the departure lounge can get crowded during the peak movement times. Both of these can be easily addressed and the latter's extension is already underway.