I personally did not have a problem. I resized my disks using gparted, ntfs and fat32 and ext4. About 8-10 times until now. It should be safe, otherwise you would see problems such as "gparted broke my disk" all around the web.
The way I see it, it wouldn't be in the ubuntu distribution if it wasn't doing its job 99%. It's a well-established program with a good programmer, well-maintained and supported regularly (by closing/fixing bugs). :)
Of course, if you are afraid of losing your data, you can always buy or ask from someone to lend you a hard drive, usb or sata, so that you can backup your important data first.
As mentioned in the comments, use a livecd with gparted (e.g. ubuntu 12.04 livecd), let it do its job and don't interfere. Also, don't do a lot of steps as a batch job (don't put a lot of stuff to do in the gparted queue). Do them one-by-one and hit apply.
There's only one risky situation, if the power goes down, as per Anwar's comment:
There is nothing as 99.99% guarantee. Sudden power failure can also
cause you a big metal plate, with no uses. Also note that: As per my
knowledge, All partitioning tools are dangerous. They always warn
their users about that