I used to feel that Gordon Brown was a very good Prime Minister when he was in No.10, until I found out just exactly how much more of the deficit he's brought down on us and how bad of a Chancellor of the Exchequer he was under Blair. I think the problem with Labour today is that they're becoming too divided between 'New Labour' (Which is just really liberalism) and 'Old Labour' before Tony Blair came into power which was what worked. The other problems are Ed the 'Unelectable PM' and that we're living in the aftermath of two previously poor Labour PMs. I think with a strong leader like David Miliband though, if he manages to bring the party together as one and outlines his vision for Labour whether New or Old, then Labour can become an effective party again.
In fairness to Brown, I gather that he didn't do much that the Tories or other finance ministers across the globe didn't/wouldn't also do, and almost single-handedly prevented a much worse European fall-out after the crash with his whistle-stop world tour. I can thoroughly recommend 'The End of the Party' by Andrew Rawnsley - it's a hefty book but you probably won't find a better account of the Labour years.