I'm doing a cardiology elective right now. I can tell you that your lifestyle is going to depend on the field of cardiology that you enter and how much money you want to make. One cardiologist that I work with usually works ~7-6 pm, but he mainly does stuff like read echos, ecg's, stress tests, and see's patients in his clinic (ie no invasive stuff). The invasive docs who I've been working with tell me that their hours are pretty hectic, one told me that he routinely works from 6 am to 10 pm! You're bound to get called at all hours of the night being an invasive cardiologist these days, since you have to split call with your group and when someone rules in for an MI or has cardiac issues, you have to be at the hospital to see them these days. Anyways, of course with the crazy hours, they are also making a lot of money (some make close to a million). Then there are always salaried cardiology positions in hospitals like HMO's or the VA hospital system, where your hours can be a lot more stable (8-5) but you take a substantial pay cut to get those stable hours too. Anyways, I think that cardiology is definitely a field where it's tougher to dictate your hours even when your in a large group because someone always has to be on call overnight and call's are usually busy calls, but like all specialties, you make your career what you want it to be. If you don't do an invasive cardiology fellowship, your lifestyle will be much more stable in terms of hours but you will make less. As a side note, cardiology is one field where I think that the demand will grow signficantly because of the aging population and the studies indicating favorable outcomes associated with earlier invasive intervention. The new generation of cardiologists don't seem to be as willing to put in the hours that their cardiology predecessors were willing to put in too. Who knows though, I was recently trying to tell one of my attendings that we weren't too far from having a statin or some other drug strong enough to knock out most coronary artery disease, but he disagreed and felt that we were a long way away from that point. Medical noncompliance will always generate a significant proportion of patients that need to be stented anyways.
As in any discipline, there is a range. Having said that, your average cardiologist works pretty damn hard. In fact, the lifestyle is often compared to a surgeons. Further, it is more difficult (not impossible) to limit your hours.
I asked a cardiologist about this today and she told me that EP's may have more regular hours then interventionalists because many of the their emergent procedures can wait until tomorrow morning or can be handled/stabilized by a regular cardiologist. However, they can have very long procedures when they do handle procedures, so it's not always predictable when you will be going home. On avg, they make more then interventionalist.