Nalgene all the way.
Last year I went on a two-day, 20 mile hike about four hours from home. I had a 3 and a 2 liter water bladder in my backpack. Somewhere along the drive the 3L bag leaked, soaking my gear with about 2L of water. The ground water where I was hiking is highly contaminated with naturally-occuring arsenic and mercury, so boiling it wouldn't have made it safe.
Luckily, being the paranoid type I had two 1L Nalgene bottles filled with water along just in case (along with several liters of bottled water). The hike went off without a problem.
Now my water bladders are just used for extra weight on the treadmill.
-Blast
If you don't mind sharing... what brand were the bladders?
I was a long time user of the lexan Nalgene bottles and still keep them around and use them from time to time. They are unbelievably durable but heavy.
I also own a 100 oz Nalgene bladder that fits my Osprey Ceres 38. I've been using it for several years and it's been dead reliable as well.
I have a couple of the Nalgene Cantene flexible bottles and they have been 100% reliable as well although I do retire them when they start looking like they've been twisted and flexed enough to make me nervous.
I tried the Platypus bottles when they were new and had a couple of failures. I haven't bought any lately but I spotted the new Evernew flexible bottles and think I will have to give them a try if I can find them in stock anywhere. If anyone has tried them I would love to hear their impressions of them.
You can see the Evernew bottles here...
http://www.evernewamerica.com/products/watercarry/watercarry.html