I've got quite strong views on working long hours* which I often discuss with people I work with. It's bad from a people perspective, but it's also bad from a company perspective.
From a company perspective, working long hours not only causes problems through stress; it also masks the true cost of a project. If everyone has to work long hours to deliver a project then the amount that the company is paying for that project isn't accurate; the project will be budgeted on resource working a standard 35-40 hour week. This can have damaging long term effects as the company will then expect future projects of similar complexity to be delivered for the same amount (or probably less), and if all the staff who previously worked long hours acting like heroes have gone off with stress or quit, then there's no way that next project will get delivered.
It also undermines the business case for the project; there was originally a predicted cost to deliver that project based on standard working weeks, against which the resulting benefits were assessed. If the project actually cost more to deliver than predicted then those benefits may be eroded, regardless of whether or not the actual costs were charged to the project and you may be doing more harm than good. If early on it's made clear that actually more resource is required to successfully deliver it, then it may be more prudent to cancel the project and therefore avoid additional spend. If no-one speaks up and just gets on with working extra hours, then the company may be wasting money unnecessarily.
[In addition, there's bound to be a quality hit if you're working 10 hour days, no matter how good you are - and what's the resulting impact on the finished product?]
From a personal perspecitve, work-life balance requires strong self-discipline; it's not just about whether you've been given too much to do, it's about being able to a) prioritise and b) to say no. Not in an unprofessional way but in a way that makes it clear that if something needs doing and you don't have enough time in a normal working week then either something less important doesn't get done, or more resource needs to be assigned. Sometimes you need to stand up and challenge the "hero" mentality that pervades some projects - you owe it to yourself, your co-workers and the company to be honest if more resource is required.
However, from a personal perspective it's important to be honest with yourself; are you working as productively as you can be? How much time do you spend dealing with email throughout the day? How much time do you spend reading BBC News when you get in in the morning? Are you using the tools available to you in the most efficient way? Are you reusing collateral from previous projects or starting everything from scratch?
There are several ways to improve productivity (for example dedicating a set period of time to email and turning it off once that time is over) and countless articles on the web that will provide genuinely useful advice for free.
Finally from a management perspective, working long hours doesn't say "dedicated worker" to me; the idea of "dedication" being demonstrated through long hours died in the early 1990s. Working long hours says to me that a person has a lack of self-discipline, poor time management and probably low productivity. Now not every manager is as opinionated as me, but do you really want to run the risk of giving that impression?
* Note: I realise it's sometimes necessary to pull a 10 hour day - every now and then as a one off, and as long as you're clawing the time back another day, then it's perfectly reasonable and sometimes unavoidable. I'm talking here about the constant and expected hero culture that is endemic to many projects.