It's "technically" fine, but large parts are also substantially over capacity during rush hour, and full of people who often are either well into a 1h-1h30 commute in the morning, or dreading one after a long day in the evenings. Once you get out of zone 1/2 things are generally a lot more pleasant.

And you see a massive difference on many of the commuter trains as opposed to London Underground - come weekend, for example, a lot of the commuter trains have a relatively cheerful atmosphere and a lot of people breaking out a beer or wine and relaxing, for example...

Travel on the Underground outside of rush hour and the atmosphere is a lot more pleasant (though Londoners will still try exceedingly hard to avoid eye contact with you).