Its quite common, I believe, but how commonplace it is doesnt matter, its how it ends up, which obviously i haven't got statics for!
A couple of RL stories:
Case 1:
My own experience- young and stupid, got involved with a very charming man, we did night shifts and worked closely together. I was in a bad place in my marriage, talks of seperation/seperate bedrooms etc, this man was so charming and we worked together so closely, things started to develop. It started a relationship that lasted about 6 months, I didnt know that he had a girlfriend who lived abroad. It all came out one day when she came to visit and found a letter I had left him.
Meanwhile, when i should have been focusing on my marriage, I had chosen to totally end it as I was so sure if I could prove to this man I was totally devoted, he would stay.
Instead his gf moved from abroad, and they now have successful careers in thier field, own home and a baby.
I later found out i wasn't the only woman he was having romantic affairs with, there were a few others of us at work- i felt beyond stupid.
Obviously I ended my marriage, but if I had seen the wood through the trees, and had never had him paying me attention at a time where i was very vulnerable, then I'm sure I would still be with my H today. I sorely regret it (exH ended up with an affair woman in the end, they have a house together and baby)
As for my career, it became difficult, he was senior to my position and I worked a position one below him. We had to do daily meetings with a team, just seeing him would knock me sick, having to work together became torture. Not only was I very hurt, but it became hard to stay professional around him and keep my emotions in check. Eventually he left, not before trying it on with me several times during the 2 month period before he left, I left a year or so afterward, the building was a horrible place for me, with alot of haunted memories (i actually feel sick thinking about it now!)
To add, my colleagues, "knew but didn't know" It changed dynamics between myself and my team.
Case 2:
My friend started sleeping with, texting, partying, spending time with a colleague (also in a higher position) this started a two year romance, my friend would always try and convince us 'it meant nothing, it just fun'
However she was clearly very invested.
She neglected her own friends and relationships for this man.
He left her crippling in debts, with an alcohol and drug problem (which she has now fixed) and in the end, dumped her for another younger "prettier" colleague, who saw him as the "family type"
Where as my friend knew too much about his darker side.
After she was dumped by this man, she had to see him everyday at work, her heart was broken, she made some pretty silly decisions including sleeping with another colleague to 'get back' at the first one.
She still works at the same place, but says she's getting over it all now, and apparently this man still tries it on.
Case 3:
A close family member of mine met her now DP at work. I dont know the ins and outs of the relationship, but I know he was a director and married, and she was in a PA role.
They started an office affair.
20 years later they are still together. They own a barn, horses, land, cars and seem happy.
But as for thier careers, relative felt forced to leave when it all came out- for the judgement she got from other colleagues, and because it was seen that my relative might get treated favorably by being in a relationship with the director, she was pretty much forced.
Within the next decade, thing were difficult for director, in the end his partners ended up buying him out of the business - i dont think the affair was the absolute reason, but i know that it was part of the overall breakdown of the business. Relations DP now struggles as self employed in hia field.
So it worked out, but not without consequences.
I'm not saying that this is what will happen to you, op.
Or that this is how it always ends up, but even the ones that are successful can come at a cost.
My advice to you is really, really weigh up how important your job is to you - could you move easily? How much contact would you still have with him if the relationship didn't work? Could you still work professionally with him? What would you colleagues reactions be if and when it comes to light? (Ie. If you or he are in a position of authority)
Just some food for thought before you get involved.
(And please mumsnet, not judgment of the 'case stories' they're all real life and I'm sure everyone involved has parts they regret, I especially do, but you cannot change the past, lesson have already been learned)