It was a Verbatim Class 6 Premium HD Video 32GB SDHC Card (sold by Amazon UK here
Taking a quick glance at the compatibility list, its behaviour was similar to the description of the 32GB Class 4 Kingston one - random file corruption (due to bad writes) and write-losses kept occurring regardless of what filesystem I was using, and it frequently fell victim to bouts of corruption that led to it being mounted as read-only, as well (I was lucky to get the current revision of my novel off of it, when it did that the last time, and it was only fortunate that it occurred when I was trying to copy everything to the new SanDisk card
). When it got into that state, and it often did for no apparent reason after a few weeks/months of normal use, it would cause the thrashing problem that I described earlier in this thread (leading to very long Pandora boot-up times), and sometimes needed to be ejected and re-inserted several times before the OS would recognise it, once booted, also.
It seems to be safe for read-only usage, as I reformatted it and have my music rips on it now (which don't need to be written to at all), and am encountering no further troubles. That doesn't make it a useful card for anyone else, though - it just means that it's not going to waste for me.