I've had pretty good luck with Parted except for one time when I ran it off the Slackware Live CD that comes with the boxed version of Slackware 8.1. The problem was that since the Live CD uses a RAM disk for the root filesystem and I only have 32MB of RAM, it ran out of memory while resizing a partition. When you run out of memory in Linux, the kernel kills the offending program. The result was that Parted got killed in the middle of resizing. Needless to say, I had to restore that partition from backup as well as my entire Slackware system. The moral of the story is that if you are short on memory, either don't use Parted from a floppy, or find some free hard drive space to use as swap. Actually, this seems to hold true for any disk intensive operations as those seem to be the ones that eat memory.
