I used to work at the Children's Hospital in my city, one of only several specialist children's hospitals in the country, so it was always busy. My main job wasn't working with patients, but I did a lot of stuff which had an impact on them, and I had unrestricted access throughout the whole hosptial, so I got to see everything.
Admittedly, you see and hear some not so nice things, and with it being children it made such things so much worse (for example, when on the cancer ward/ICU or when you hear a child has died), but when you have a good day, that more than makes up for everything else... when you know, you've done something that has really made a difference (for example, in my job, I could have stopped operations, and had to deal with 'urgent' requests which had to be sorted so there was no impact on patient services), it's a great feeling. There is nothing like it - you feel so satisfied.
I would say that, without a doubt, the good days will more than make up for the bad... and the bad days, are character building, if nothing else. Plus I would further say the NHS is a great organisation to work for... there is something special about the people who work for the NHS - and that's the same with everyone I've met who works for the NHS, not just in my hospital - without these great staff, the NHS would collapse (I can't begin to add up how many extra hours I've done for no pay, or how many scarifies I've made for my job and my hospital, or the sort of things I've seen done by friends and colleagues too - such as working from 7am till well after midnight, without a break). You'll meet some great people.
Anon - if you have any issues you wanna discuss about working in a hospital, you can PM me. Ditto to anyone else.
EDIT: With regard to the anxiety I had that a little at first, when I started working for the NHS. But I got plenty of support and everyone is really nice (again, it's the people that make the NHS what it is). I had it even more after I got a double promotion. Everyone (my bosses) were so confident that I could do the new job, but at first, I felt I was in so over my head, and I nearly turned around and said "I can't do this". It was so much responsibility, both in terms of patient related services, decision-making and spending high amounts money, I was just so afraid I was going to make a big mistake and it would have an impact on our operational/patient services, but as soon as I realised that I was good at my job and became confident at it, I was way ahead of my colleagues and everyone (people from outside of my dept and my bosses) recgonised that and started turning to me for things. So it worked out well for us all.