This game is the definition of a simulation- it's very detailed and true to life. If you're into these kind of games then you'll enjoy it, if you're expecting an exciting game with heart pounding adventure then you'll be instantly bored.
The game does feel like it needs a lot of polish - the interface is clunky - you have to hold the 'shift' button whenever you want to interact with any controls in the cabin - although there are keyboard shortcuts for some controls.
Once you get used to the controls though driving the train is pretty easy and it's pretty cool seeing the different parts of London as you drive through - except of course when you're in a tunnel (which is a lot of the time) and that's the main problem with this game really, although the detailed and accurately modelled train stations are nice to see (which you can explore on foot as well).
There are 2 modes you can play - missions and 'drive to schedule' which is basically free mode where you get to choose your route, time of day and weather. The missions are pretty much similar to train simulator - drive so and so route and keep to schedule - with emergencies and whatever else thrown in to challenge you.
The game is very harsh when you do something wrong during a mission (like speeding) - it will instantly end the mission, instead of giving you penalty points like train simulator.
I also had issues when first playing - the framerate was VERY low, as in 10-15 FPS. My PC can run Skyrim on ultra settings with a smooth framerate, so it was a bit surprising.
I checked the graphics settings and turned the anti-aliasing and bump mapping sliders all the way down which made the game smooth and playable (I kept draw distance to 'very high' though which you'll want to do unless you don't mind having buildings pop out almost in front of you). After doing this the graphics still look impressive enough for me.
I recommend it if you're really, REALLY into your sim games, if not you might want to save your money - you'll likely get bored after a while.