Name changed as will possibly be recognisable from this.
Dh is just finishing his F1 year. He got on the 4 year course having just finished a science degree- afaik you need to be a graduate, preferably in a science, to get on the 4 yr. Perhaps this varies by University?
It was hugely competitive (hundreds and hundreds of applicants, 20 posts), and they were basically expected to fit the 1st and 2nd yr lectures in to one year- there were no lectures set up just for them. They had both years schedules and worked it out between them, often attending different ones which were on at the same time and then swapping notes. The first year was very full on.
After that it did ease quite a bit, as other posters have mentioned, if you have worked in the 'real world' it's not as much of a shock as for those coming from only education. Also lovely getting the majority of the school holidays off (no half terms though).
Now he is an F1 it is harder in many respects, easier in some too. He doesn't have the endless guilt of study weighing him down (if he wasn't studying he felt guilty), although he still does study in his time off it isn't quite so intense. The hours are huge though. He has to be in by 7am and is rarely home before 7pm at leadt 5 days a week. Most weeks there is a 8pm or 9pm finish in there which doesn't usually finish on time, and then I think every 3rd or 4th weekend is fairly normal (this is mostly a shorter day though) with no extra time off.
For example, he may work 12 days in a row with 3 of those being 12 or 13 hours, all the rest being 10 hours and virtually none of them finishing on time. He would then get 2 days off, and be back for 5 more at least 10 hour shifts. I'm not joking when I say that he doesn't see the children awake for a whole week sometimes. Also has other things which need to be done during year- audits, presentations, other things to add in to your CV so you can follow the career options you hope for.
However, this is the busiest bit he is on, the next placement he will not be doing quite so many hours as shifts start later and no nights. Hours depend on which area you are working in and what pay bandng it fits in to. I think that this also differs by hospital though. Most hospitals we looked at did a basic banding for at least one of the 3 or 4 month placements, and this will have basically 9-5 Mon to Fri working hours. The other placements will then be higher pay and more/more unsociable hours.
Again this may differ between hospitals, but it is quite difficult to book leave and occasionally to find out what your shifts will be before you start a new post. Leave for DH is divided up so he can only take a prescribed number of days per placement, this means you can't fit in a 'proper' holiday, not a big deal for us, but maybe for some.
We have 3 young DC. Absolutely no way could I have done what he has done. You must have to really want it to go through all that.
He does love it though and is very committed, and left school with nothing, so has done incredibly well 
Good luck with it!