I bought a Sandisk class 6 and the difference over my Sandisk class 2 cards is amazing. Both report correct transfer rates in the SD card tester app and the class 6 is 3 times as fast. Using the gallery is greatly improved and because I used a2sd+ the loading of apps is much quicker as well. So I completely disagree with the posts above - just check the I/O differences of a few cards and the difference is obvious. I would say buy a brand you trust (from a reputable source) and the speed will match the class on the card. I imagine the people above have bought cheap Chinese fakes or something.
I assume the OP has seen that hot deal on a 16gb class 10 card that is going around? I'm thinking of getting one as well.
EDIT: Actually I'm probably being a bit hard on krooton there, some lower class cards are known to transfer higher than their rating suggests but it's by no means a given as all my class 2 Sandisks only transfer 2MB/s whereas my 6 is 6MB/s. Best way is to test the speed and if you're only getting 2MB/s then a class 10 will be a big improvement, assuming the card is for real.