There's no logical reason why those 'space bags' should reduce the overall volume of clothes when packed. When you fill a suitcase to the brim and force it closed, you are forcing any extra air out of the way to compress the clothes down, and unless you are packing a down parka or pillows, the minuscule amount of space you save through compression comes at the cost of the thick plastic used to construct those things, and the reduced amount of flexibility in packing. Basically, if you're trying to pack stiff blocks of plastic into a suitcase, there will be tons of unused space around them. If anyone is able to pack more using those vacuum bags, then they must have been awful packers to begin with.
My method? Neatly fold dress shirts and items that need to stay unwrinkled and place them in the bottom of the suitcase, and fill the surrounding space with socks, underwear, t-shirts, and an extra pair of shoes. Pretty easy to pack and unpack.