It really depends on the circumstances.

For a night train, it's often a better option, as, if I can get one at midnight, it basically buys me back the evening at home.

For a day train, if I can work from the train, then 4-5 hours is fine (particularly on the ones that have Wi-Fi connectivity). Beyond that and it depends on what I'm doing around that time.

It'd be great to have more sleeper trains running from London, particularly through the Channel Tunnel - so much nicer than the old 4am start.

I wouldn't consider driving an option, really - I don't drive where I don't have to. Plus, it's lost time, really - 4 hours on the road listening to music vs 4 hours on my laptop on the train in peace and quiet...