I use my SD card slot all the time, its how I move my pictures from my camera to my computer.
As for a permanent disk solution, I saw in some thread (not sure if it was here), that running some apps, can cause it to heat up, do the high volume of read/writes Not sure if anyone has saw this issue.
I've seen reports of heating with small USB3 flash drives and also some mSATA SSDs. It seems that the combination of high speeds and a small silicon chips for large storage capacities, results in high(er) temperatures.
I suspect that it would be prudent to reserve these SD card storage devices for applications where the files are seldom written. Music files, videos (but not video editing), ebooks, user manuals and other read-often documents would be obvious applications.