It's crap.

At least, in my opinion --- I live in Switzerland now, so my standards are... different. But, basically, overpriced, unreliable, dirty, and slow. The only places where the train system actually works well is the south-east of England, where it's horribly expensive. I used to live in Reading; a peak time day return into London, which is maybe half an hour a way, costs about £40.

Bear in mind that I know perfectly well that Britain's rail network is amazing compared to, say, America or eastern Europe. But the local perception of it is that it's bad. (Not helped by several rounds of botched privatisation, too.)

Incidentally, I'd love to know how to get from Ireland to Great Britain by train. Do they float?