GP using the word 'troll' could have been a cleverly constructed joke, seeing as prior to being bought by Nokia, the employees at TrollTech were known as 'the trolls'. Not sure I believe it is a joke, though.

I think you accidentally a word.

There is no myth. I benchmarked it - under sustained writes, reads go very slowly indeed. Large sequential writes get very poor throughput as well.

On Windows > XP, not at all. On XP, a little tougher. On OS X and Linux, much better, because you don't need to install a new piece of software to access, and native UDF is much faster than ntfs-3g.

It will probably be much easier to develop an exFAT driver in kernel space, because it's very similar to FAT. This will, again, give a performance benefit relative to ntfs-3g

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Basically, everything even half-modern except Windows XP supports writing, and everything half-modern including windows XP can read v2.0 and down. See the relevant Wikipedia article for more specifics. There are third-party drivers for XP, so your best bet is probably to either:

a.) Format it as UDF 2.0 or less, and put a write driver for XP on it. This requires admin access to the XP computer.
b.) Split it into a small FAT partition and a bigger UDF partition. If you can't write to UDF, save files back to the FAT partition instead. This variant works on computers where you have no admin privileges.
c.) Wait until a read/write driver for exFAT hits OS X and Linux (Linux already has a read-only driver for exFAT)