I am a student, and so not a commuter, but I do use South Eastern trains to reach London and my friends who live in neighbouring towns. The service from Sevenoaks I've found is generally reliable, though there are as ever, problems with delays.

Trains themselves:

The new trains are much nicer than the old ones, much cleaner and generally made up better. However, they can be very cold in the mornings and during the winter, and there is a problem with cleanliness. There are always hundreds of old newspapers and people's rubbish left around. Another issue is seating: there is a lot of space taken up by first class seating, which is hardly ever needed! There is usually a trolley service though, which is nice.

Cost:

The price of tickets rises constantly it seems! There are discounts to be had: young person's railcard, family railcard and senior savers and of course it's cheaper to travel off-peak, but generally an adult travelcard sets me back twelve pounds! And just two stops is about five pounds: not really worth it, but what can you do?!

Service: The trains are pretty frequent, especially to london and the fast trains are very useful for getting to and from London.