I just ran my first trail marathon (Salida, CO) last weekend with an UD Wasp Hydration Vest. While I absolutely loved the vest during training runs of up to 25 miles where I carried EVERYTHING I needed ,I hated it during a race. Naturally I didnt' want to be carrying 3 L of water in a race situation, so I was relying on the aid stations every 5 miles. While the vest is incredible for in between aid stations, I came to DESPISE it when I got to the aid station. Beforehand I had practiced to try to get fast at removing and filling the bladder inside, but when really rushed at a race, it didn't go so well. Every time, I would be there messing with the bladder ,closing it, etc, and guys would come from behind and dump a few cups of water into their handhelds, and pass me. Then I'd have to work hard for the next mile to pass them again. I still love the vest while running, but have already begun using an UD handheld (which I already love) instead of keeping water in the Wasp.
I'm running a 50 miler (Fruita, CO) in a month, and plan to carry the handheld as well as my vest. This time I most likely won't keep any liquids in the vest, but it will be good to hold my camera (essential), as well as gels and random foods and keep them out of my pockets.
Best of luck with the trail running.
http://ultrapayne.blogspot.com