I was having trouble with the durability of the spring loaded eject mechanism on the SD slot. If not careful it would wear out rather quickly and eventually the SD card would either get stuck in, or out (meaning you couldn't get it to stay in).
After looking at my USB card reader, which doesn't give me any problems, I decided the answer was to ditch any sort of spring loaded anything! I couldn't have something I made go out only to have people upset because it broke after a few weeks/months.
I looked at a few different places and bought a couple different SD slots so that I could test them out and see what I liked best. These are my two favorites (I kind of like the shorter one best).
I will put a rounded cutout in the PCB above the top part of the SD card so that you can just grab the end with your fingers and pull it out (or push it in) without having to worry about some spring eject thingy breaking!