I feel quite happy with SSE, mostly with the better graphic quality overall in the game and the many ships/boats.
I enjoy Sydney and Dover-Calaise very much, Bora Bora is very nice looking but i be a bit bored by enviroments thats mostly mountains and woods, Antarctic is interesting but i feel it to be too little to do in that enviroment with only one poolarbase and the ice feeling as one big block (but i have not sailed around in Antarctics so much so its just my feelings that more passages and two bases to sail between would be a big difference to me here).
The old enviroments is a lot better than before and the weather system/nightview is great so overall i feel SSE is a great improvements in the areas i am interested in.
To be honest i have not used the missions yet, for me they have sounded too time consuming and complicated for me to test out until now but when i have got tired of free room trips i will take a look at the missions also.
The downside of SSE important to me just now is just a few: the Steam thing, problems for new users to install the game, the map function/no size slider as in 08, the grey marking of ships in the maps (hard to see).
I have also seen som crazy behaving AI ships even after the last patch.
But no big complaints anymore.