I agree with Ash - there are both good and poor versions of each. Quality really counts here regardless of leg lock type.
I have an older Bogen 3021 tripod and a Bogen monopod (both with flip-locks) that have never given me a bit of grief reliability-wise in over 15 years of use. My only complaint is that they tend to snag things when I'm toting them around.
I also have a Giottos MT 9371 and a Gitzo GT5541LS with twist-locks - both of which function well. The Gitzo in particular is very nice to deal with. Obviously I don't have the amount of mileage on either that I have with the Bogens but they do seem well built.
Edit: I use all of the above outside in the wilds of the wonderful sandy and swampy Gulf Coast of Florida with no problems.
Last edited by jem; 07-06-2009 at 03:30 PM.
Reason: Additional info