I left practice in Novembver 2004 and have never looked back. I was never killed with hours to be honest, but that was a function of:
(a) the assignments I worked on
(b) my working style/practices
Many of those around me worked until 10/11 pm and Saturdays as a matter of course for 3/4 months of the year.
I now work in finance/compliance in a large blue chip in the financial services industry. I had enough of practice, and it was only going to get worse (less capable junior staff, less partner time available, more onerous company law requirements on clients, more and more senior staff leaving), so I jumped ship before the money on offer became harder to get elsewhere.
My advice: if you can get the same money you are getting now (I actually ended up getting more, but was willing to take a slight pay cut), and can find a job in which you will be reasonably happy, get out now (the jobs market for qualified accountants is as strong as ever). I have no grudge to bear against practice, I just think that a culture of long hours has built up to such an extent that people feel they have to be there until a certain time every night, regardless of how busy they are.
If you stiil want to stay, consider this. I never did as many hours as anyone else in and around my level, and I was never seen as a slacker by management and partners, in fact I consistently received the highest appraisal ratings available. Don't think that your superiors equate long hours with dedication or anything like that. They are far happier if you get the job done in as little time as possible.