I only have experience of one of these operators. PAS/MAS are the other main player and don't forget Aeromega - a smaller player but a really friendly and professional bunch, but with limited floater requirements. Floater rosters are pretty much by definition changeable and vastly variable. You are there to replace line pilots due to leave or sickness or pilot shortage. In well regulated households there should be reasonable notice of leave, but sickness, compassionate leave etc. mean that many shifts will be doled out with very limited notice. A commercial company providing pilot services makes money by pairing back on staff - if it has fewer floaters it might even make some money!
It is all controlled by the FTL system of course. The night shifts bu**ers up the thing, as does positioning, especailly when there are 12 hour shifts involved. Some aspects of the FTL scheme must be followed (cos there is a paper trail) - other things can be "ignored", in order to "make best use of the resources". Forget 14 days or 7 days notice of the roster - you will get rostered for some shifts with just a couple of days notice (or less). I know its not "legal" but its what happens, and you cannot really blame them, if they only have a day or two's notice of sickness or whatever.
Expect a vast amount of self-drive surface travel and a modest amount of flying, so don't do it if you don't like motorway driving or a are keen to acquire lots of hours. Don't expect to plan with any certainty any social engagements unless you take leave to cover it, and you will typically only have 4 weeks (20 days).
BAS and Premiair (Utilities) are likely to be quite different as BAS have only one police unit (the rest being HEMS and other daytime flying).
In my experience most floaters don't last that long (12-18 months). They will often go for a fixed line job or some other role with the same outfit or (for police look for a direct employed job). However, some have lasted a longer period (up to 7 years or so). I had to give up cos my dog attacked me every time I went through the front door.
The up side is that you get some variety and you can see just how differently different units operate. A good way of learning about what is going on too, which can help for the next move.
There are additional differences depending upon whether you are employed or freelance too - it changes the balance of control. As an employee they own your soul - as a freelancer you can end up with no work especially at certain times of the year.
Hope that helps.