pex is the best "alternative" to come along so far. it has none of the bad characteristics that previous poly piping had, such as quest.
as i'm sure you know, how a crimp is made plays HUGE part in how reliable pex is. if you take your time, make a nice crimp, it can be a good joint. if you're sloppy and crimp the ring off center or oblong it, it's probably going to leak eventually. i personally am not a fan of any mechnical connections, but as far as they go pex seems to be okay.
pex can be done nicely, but the problem is usually the installer doesn't give two shits about the finished product form/function, as long as it works. like i said before, people don't know the difference either. they don't know their shower arm isn't supposed to jiggle around when they adjust the head, they think it's normal for stops on the sinks/toilets to be coming out of the wall at a 45 degree angle, they never go under the house so it's okay if it looks like spaghetti under there, etc.
also, pex really hasn't been around long enough to fully determine how good it is. it's been out since the early-mid 90's, but not really used widespread until recent years. with the boom of the tract building market, pex has come with it as well as so many other time saving materials. it's holding up well in my experience though. the only places we really see problems are with the brass fittings on well systems (nothing holds up great to well water except cpvc) and poor installs.
copper is still the end all be all for water systems. you will pay dearly for it though, both in materials cost and labor. it makes for such a nice looking job though, it's quieter, and done right shouldn't have to be touched for 30-40 years if not longer.
