Another viewpoint - specifically for anniemac re partnership (although caveated in that I'm not a lawyer - I'm a partner in a Big 4 accountancy firm).
Don't think of partnership as being more of the same job you are currently doing - it's quite difficult to explain but it is very subtly different. You will still be doing a lot of the same type of work, but the job is different - it is demanding in very different ways but also rewarding in very different ways (apart from the obvious financial benefits).
In my experience the biggest difference is about decision making - on my clients and my projects the buck truly stops with me - and as a consequence I have to make judgement calls that could have serious implications for my clients and me if I get them wrong. This imposes a very different type of work pressure to the general "too much to do too little time" work pressure that is endemic in professional services.
It's different because decision making really is about quality of thought not quantity of work - so as a consequence a partner's job does become more flexible. I still work long hours (by "normal" ie non City reckoning) but they tend to be when it suits me. For example, I'm going to be in late tomorrow because there's a parents meeting at dd's school at 9am which I want to go to.
If you are the sort of person who thrives on the challenge of ultimate responsibility then partnership is great - I suspect it helps to have self belief and strong controlling tendencies. And it's the second of these I think that can cause part time careers to falter. if you are able to effectively switch off mentally from the office during working hours then you perhaps don't have the "need" to achieve promotion. And, in my firm and probably most others to get promotion you have to be not just technically and commercially qualified but there's also a certain emotional issue as well which is difficult to describe but is sometimes labelled as "commitment". It's not just that though, it's something that you can't pretend - you either really want it or you don't.
If you can find too many reasons not to go for promotion or excuses why you won't get it - then it almost certainly isn't actually what you want. Life is full of compromises - every decision you make to do something is equally a decision not to do something else. The secret is to know what is right for you - not for your friends, colleagues, strangers on an internet forum. You're leading your life not theirs.
I think I've got it right for me - that's not to say I don't have bad days but I'm fundamentally happy with my family, job, house, finances, life etc. And, as a result, I don't feel threatened by other's choices - it may make me question my decision, but it hasn't (so far - and I'm pretty ancient now) made me wish I'd made any different choices. And if that's not having it all - it's certainly having enough for me.