Let me start with a comment on other reviews. I think Aussies went into this looking for a comedy that speared their politicians and are disappointed that this (as dar as I can tell) doesn't do that. In common with the other Brits reviewing this however, any comments about Aussie politics are relatively moot anyway and we get to focus on the characters.
And if you come in with the understanding that this is not stinging political satire as Yes (Prime) Minister was, but a jet black farce propelled by grotesque characters with few to none redeeming features, then you can relax and enjoy the ensuing ridiculousness on display without caring whether it scores political hits or not or approaches any kind of realism.
It is very funny. It does take a while to build into it with the first half of the first season being much more subtle and refined, it raises a smile and some titters, but as the show goes on, the humour broadens out and many of the episodes leave me very merry.
It's not perfect - there's a big shift, not just in tone, but also in concept between series 1 and 2 as the Ex-Pm returns to the hustings in an attempt to be more on the nose. Although series 2 takes the farce to even greater heights, it does kind of defeat the point of calling the show The Ex-PM. Also a couple of the characters just don't make the jump. Both Ellen and Rita may as well be entirely different characters between Series 1 and 2, through no fault of the actors involved.
Does it matter? Not really. The show more than earns its way. There are lots and lots of sitcoms out there that don't. Well worth a watch.