They're a contractor, nothing more and nowt less.
I worked for them a long time ago, but I know many people who recently were and still are employed by Serco.
Generally the feedback from UK staff is pretty positive, they do look after you, salaries were OK and the pension scheme used to be final salary (although I'm not sure if new joiners are allowed in now).
The good thing is, that despite not having many UK contracts left, those that they do have are pretty long term (albeit mostly military based) and they need as many atcos as they can get on their books, so, as has been mentioned, they're always willing to train staff.
Additionally, if they are faced with serious competition on a contract re bid, they have the financial muscle to put you on fully paid gardening leave for a long time just to screw up the chances of someone else employing you and getting the contract - they did that at Wattisham to great success.
I suppose the other advantage that some may see, is that you are working for an organisation within which you can move on either to another of their airports (in the UK or abroad) or to another life within the company itself.
From what I can see, you either join Serco and move on outwith the company within two years or so, or you stay with them for the majority of your career.