How well does Ubuntu support HFS+ (i.e. Mac formatted) filesystems? Can it read and write to them, and are there any potential problems to watch out for?
The kernel supports basic read support - however, journaling support is nearly non-existent which is needed for write support. By default most modern Macs (which utilize the HFS+ Filesystem) have Journaling enabled. For write support you'll need to first disable journaling on the HFS+ Partition:
sudo diskutil disableJournal [VolumeName]
Then, if needed, re-enable with:
sudo diskutil enableJournal [VolumeName]
However, as a warning - Journaling is an important part of any Filesystem which implements it. Disabling it in Mac OSX is not recommended and can have adverse affects. There has been work to include journaling support for the HFS+ driver in the kernel - but that it still some time away.
SuperUser has a great answer for mounting HFS+ partitions in read/write mode here:
Looking at the HFS+ driver in the kernel, it seems to have read & write support, but I have absolutely no idea how stable/useful/etc. it is...