A lot depends on how well you build the cooling system. When I started mine I was also concerned about leaks but six months on I've not had one leak yet anywhere in the system.
Here's an older picture of the Pipework, it's a bit neater now.
<b><A HREF="http://www.zytec.worldonline.co.uk/pictures/jpgs/water-cooled.jpg" target="_new">Water-Cooled</A></b>
I'm also cooling the CPU, Vga GPU & Ram, Mobo Chipset, and HDD now
More details in my sig link and these Pics of the updates since.
<b><A HREF="http://www.zytec.worldonline.co.uk/pictures/jpgs/asuswcmk2hmed.jpg" target="_new">Watercooled Asus GF3 MK2</A></b>
<b><A HREF="http://www.zytec.worldonline.co.uk/pictures/jpgs/hdd3small.jpg" target="_new">Copper HDD waterblock</A></b>
I recommend you get everything together for a watercooling system and test run it all for a week on the bench. most of my blocks are homemade and I test them out with compressed air a 2bar, (28psi), with the block immersed in water. If it wont leak air at 2bar it definitely wont leak water. I also make sure the pipe I use is almost impossible to get on the barbs which will insure it can't pop off, quite a few leak failures are caused by pipes not being tight enough. (My rule is you should never be able to pull the pipe off the barb however hard you try, cutting it off is the only way)
As said it will be as good and leak proof as YOU make it, That is not to say I will never have a leak with mine but along with all the above and the fact I will replace the pipes once a year or whenever I have to remove them for upgrading the PC, (whichever is first), is the best leak insurance I can humanly do.
I just have the PSU to do and then there will be no fans & silence
<i><b><A HREF="http://www.dwpg.com/stealth/1/" target="_new">Stealth Cooling Project</A></b></i>