When I asked this same question at a company I used to work for I was told that they were generally leary of mechanical type joints. After I worked there for a while I found out more about why. They had a line that was put together with dresser couplings (compression couplings) and over the years had experienced some problems with them, mostly seals drying out and leaking.
In my opinion, and take it for what it's worth, ZapLok or other similar systems are alright for systems that are expected to have a fairly short lifespan, but for systems that are expected to be around for the next 100 years its probably better to go with a butt welded line. I don't know exactly where I'd draw the line of "short lifespan", but my gut feeling is somewhere around 10-20 years. Maybe in another 100 years we'll look back and still have ZapLok pipelines in service, time will tell.
Previous comments aside, I have heard good things about the current ZapLok systems and I've seen them installed in gathering situations several times, but I haven't actually used it on any of my own projects. An advantage that a product like ZapLok has over a dresser style fitting is that the epoxy used for the seal doesn't dry out like a rubber seal will so as long as you don't disturb the joint once it's in place it shouldn't leak.