The higher the HCA(hydroxycitric acid) concentration, the more mileage you are going to get out of the supplement. All brands will contain some amount of the extract from the actual fruit, but the amount of the active substance can vary a lot.
There are plenty of fly-by-night products around(pretty much all of the ones in local stores) that only offer a very subpar purity.
Often times such supplements(like ones sold at WalMart) only contain 20 or less HCA- even when they are marked as higher(this was the conclusion of a recent study). Such low-grade products(like the ones from GNC usually) are basically scams, replete with fillers and additives and dont do much to give you the true benefits of the natural extract.