Micro SD cards, just like the larger cards before them, do have major differences in performance according to their class ratings, but that performance won't necessarily be seen in certain situations.
For example: A DSLR that has the capability to fire off several shots a second will actually be noticeably faster if it has a higher class memory card, because the faster card allows the camera to save image data at a much higher rate than a slower card would. With the higher class card, the camera doesn't have to pause and wait for its buffer to empty before taking more pictures, it just keeps shooting. The difference here is that your typical Android phone isn't likely to put that big a load on the write capabilities of its memory card.
Most of the time that you spend waiting for a picture to be saved on a phone is usually the phone processing the image, not saving it.
A typical Android phone's Micro SD card might be a Class 2, meaning it's guaranteed to write at 2MB/s. Most companies actually make the cards to exceed that expectation, sometimes by quite a bit. If you take huge amounts of pictures with your phone, a Class 4 Micro SD card, which is just one class higher, might benefit you. It won't exactly be a life-altering experience, but it could speed things up if your phone's camera takes pictures at something higher than 5 Megapixels. Most Android phones don't do that.
If you use the USB Storage feature of your Android phone pretty frequently, then things are a bit different. When plugged into a computer as a USB storage device, the full capabilities of the card are used to transfer data—so the faster the card, the faster your data transfers. Most people might plug their phones in to do this once a month to switch out their music collection, so it's no big deal. If you use your phone like this regularly, then the cost of a higher class card may be worth it.
Update:some Android users make use of the Apps2SD or DATA2SD features available on some aftermarket ROMs (running apps from the SD card to save internal storage space). In that case, a faster card can make a major difference. For heavy Apps2SD usage, it wouldn't be a waste to get a Class 6 card—and it's pretty much required for DATA2SD.
If you've had either good or bad experiences with certain Micro SD cards, share them in the comments so others can learn from them!
