Andrew Flintoff throughout his career as a whole was somewhat over-rated, but what caused people to rate him that highly was what he did in a 2 year spell between '03 and '05, with the Ashes being the climax of a magnificent run of form. Between July '03 and the end of The Ashes in '05 Freddie scored 1790 runs at 42.61 with 3 centuries, and took 102 wickets at 27.94. Those are the stats of a world class all-rounder, and a peak where he could have made the England side on batting or bowling alone. Interestingly, his ODi stats in that time period look superb too. 1140 runs at 51.81 with 3 centuries at a strike rate of 98.19 and 44 wickets at 21.22. For those 2 years, I'd say he was the best cricketer in the world.
Freddie's problem was that he was picked FAR too early, and then after '05 he only managed 1 more epic series, in India, but was plagued with injuries and couldn't put together sustained spells of cricket due to his knee problems. In his prime he was the best all-rounder in the world. Kallis was excellent, but he's always been very much a batting all-rounder, and you could never really say Kallis could have made it into the South African side on bowling alone, with Flintoff you could.
To just dismiss Fred's accomplishments with ludicrous statements like "he sucked" is just laughable. Flintoff has bowled some of the greatest spells of quick bowling I've ever seen. The '05 Ashes was filled with them. There's that 1 over where he nipped out 2, and was all over one of the best players in the world in Ponting before getting him to edge behind, it was a magnificent over, and probably the best over of fast bowling I've ever seen, and some of the innings he played were ridiculous. When Flintoff had form and confidence with the bat he could take anyone apart. It's a real shame his purple patch didn't last longer, if he had managed that for his whole career he'd have gone down as the greatest of all-time. What a player, and what a man.