First come the bad bits: London South Coast does have a few flaws here and there but it doesn't spoil the add-on much. Firstly there are a few trees in the forefoot, especially just south of Purley, where they cover up part of the northbound fast line.
Clapham Junction station seems empty, which is stupid, because it is in real life one of the busiest train stations in the country. There are not enough activities from London through to Portsmouth (only 2) and a few activities with trains dividing (like in the old Brighton to Portsmouth add-on) would have been nice, but that's the bad stuff gone.
Overall, I think this is a great add-on for the legendary MSTS. Gatwick Airport station is very realistic, with the aeroplanes landing and standing at their gates which you did not get in the old London to Brighton. The new Arun Valley line is absolutely fantastic, with Horsham station being very detailed. Also, unlike the old London to Brighton and Brighton to Portsmouth add-ons there are no errors, which makes a run on LSC very smooth indeed.
I love the rolling stock, which includes trains from the slam door class 421s all the way through to the modern Class 377s. The trains are very realistic, for example the Class 171 Turbostar sounds exactly like the real thing.
They have even managed to create a realistic approach control system that leads up to many of the diverging routes, such as to Cosham and at the Ford triangle.
Overall I think this is among the best MSTS add-ons that I have bought, it is a must have for any lover of MSTS. Highly recommended.