I've been twice, and both times it's been harmless cysts. The last time, the told me they expect they'll be seeing me again, as there are several cysts which are likely to cause more lumps in the future.
In my appointments, they tell you to allow up to 5 hours, so bring a drink, snacks and something to do. There's generally quite a lot of sitting around and then being called in to see various people. My hospital has a special women only waiting room which is nicer than the main one.
For my appointment, I start by waiting then get called in to see a doctor who takes my medical history and feels the lump. In my last appointment she then called in the consultant who also had a quick feel. He told me that a surprising number of women don't get past this stage as their lump is just normal breast tissue.
In my case, first time round I had a mammogram and then an ultrasound with waiting in between each one, and was then called back in to speak to the doctor who went through the results and told me that it was harmless cysts.
Second time round, I had an impressions and then had fluid drawn from cyst, partly to make it shrink but also to check the fluid, as bloody fluid can also be an indication of something more serious. This was mildly uncomfortable, but only in a skin-prick way - easier than giving blood, or having a vaccination. This was all explained to me at the time.
I was then called back again to the doctor who again went through my results with me, told me what to look out for in future and then sent me home. I had a morning appointment, and spent about two and half hours there.
I obviously haven't gone through the stage of getting bad news, but that should give you a basic idea of what some of the tests are like.