I think the lever clamp locks like you see on the Bogen/Manfrotto are easier at first, but once you get used to them the Gitzo twist locks are quite fast and I prefer using them. I have a Gitzo CF tripod and an aluminum Bogen/Manfrotto monopod. Personally I don't have a strong dislike for either system... except under some conditions the Bogen/Manfrotto lever clamps can be to loud when you close them. If I'm setting up in a quiet church, or trying to avoid spooking the wildlife I'm out there to photograph, I'd rather have the silent twist locks. The clamps on my Bogen/Manfrotto monopod are very loud by comparison, but maybe it's because of the aluminum... perhaps they are quieter on a CF tripod or monopod.
In my experience twist locks make minor height adjustments much easier since you can slowly unlock a leg section so that it takes a bit of force to slide it up or down. With the lever clamps things are either engaged or they aren't... so you better make sure you have a grip on your gear when you release a clamp to adjust that leg's length.
Once you get used to them the twist locks are just as fast in my opinion (or close enough to where the difference is insignificant). The trick is to extend the middle section and lock it in firmly so it doesn't twist when you unlock the lower section. Don't go as firm on the lower section when you lock it and you'll be able to unlock the lower section without unlocking the middle section. Sounds confusing, but really simple once you do it a few times.
As far as twist locks being more prone to contaminants... that can and does happen... although I don't think it is as common of a problem as the advice you quoted implies. I've photographed on many beaches and never had sand infiltrate the twistlocks. I never laid my tripod down in the sand though. If it happens, here are some instructions on how to clean:
http://www.earthboundlight.com/phototips/gitzo-maintenance.html
I haven't had any problems with water, but I have yet to submerge my tripod in anything deeper than a few inches. If you submerge your tripod past the first section twist-locks in a particularly muddy or silty body of water I could see that being an issue, but you'd only have to clean the three bottom locks. I can see this being an annoyance with lever clamps as well since the sand could get inside the legs making them more difficult to slide up and down, but they are probably easier to clean. I don't think it's a problem worth worrying about on either system unless you plan to spend a lot of time submerging your tripod in water or mud. Think of all the landscape photographers who use and recommend Gitzo tripods. If this were a larger issue they wouldn't be recommending them. Of course there are other nice tripods and clamps out there as well... including the bogen/manfrottos. Just don't let the twistlocks scare you. There are more important considerations when choosing a tripod.
Sean
