It has become fashionable for various internet gurus to decry the unreliability and/or onerous maintenance requirements of this gizmo:
What you see here is a Shimano LX rear derailleur, "Rapid-Rise" (i.e. low-normal or reverse-action) version, from about 3 years ago. It started as HC inventory way back when, but came to me this time in used-condition (from a coworker's bike?) and is now installed on the Pugstigator. At HC, we occasionally use road derailleurs (Tiagra, 105, Ultegra, Dura-Ace, Campagnolo, etc), but for the types of bikes we usually build and fix, mountain bike parts generally make more sense, and for various reasons, Shimano is the brand we use the most. From the economical Alivio/Acera all the way to bling-bling XTR, I have used them all on personal bikes and customer bikes, and I think it's safe to say that Shimano makes rear derailleurs that work well, are easy to adjust, and that take the use, abuse, and neglect that folks tend to heap on them. Anytime you read an opinion that derailleurs are inherently unreliable or require tons of maintenance, you should immediately question the credibility of the author of that opinion.
Of course, derailleurs get a bad rap from people who haven't used one made in the last 20 years, or from those who attribute minor mis-adjustments to an inherent design flaw. Of course, a legitimate case can be made that the derailleur, being exposed/dangling as it is, is subject to getting bent, say, if you back your car into it or slam it in the trunk (this happens A LOT). The case could also be made that derailleurs can't shift while coasting or while the bike is stopped, which is a real drawback in certain applications (especially cargo bikes). And, last but not least, it's a strange-looking mechanism, and not every cyclist understands how it works, how to adjust it, or how to keep it reasonably well-maintained. (if you are a local customer of ours, and don't know these things, we will be HAPPY to give a free hands-on lesson - just ask).
Regardless of whether the problem is with the equipment or with improper use/abuse, it seems clear that much of the trend of the past 10+ years toward single-speeds, fixed-gears, and more recently, internal gear hubs (IGH) is the result of folks being frustrated or intimidated by derailleurs. Personally, I think single-speeds make a lot of sense for a city bike in mostly-flat Minneapolis, and I really enjoy riding fixed-gear. I could also list a number of applications where the IGH has some real advantages over derailleurs, primarily: you can shift at a dead stop (imagine riding a loaded Big Dummy up to a stoplight only to realize you're in a too-high gear) and often there are benefits to having a chainline that doesn't change. If you come to HC looking for a bike for which an IGH is the best choice, I will certainly suggest that option, but I will also make sure you understand not only the advantages, but also the drawbacks (yes, there are drawbacks, and they are significant).
Yesterday I received a call that started with some words that are now familiar to me: "I really like the idea of an internal gear hub..." Of course, the "idea of an internal gear hub" that the caller was talking about is the mythical drivetrain that requires almost no maintenance, is guaranteed to be reliable under even the most extreme circumstances imaginable, and almost never needs to be repaired or even adjusted. This is simply not a realistic expectation of any IGH system, though the fantastic (and fantastically expensive) Rohloff comes close. Anyway, he went on to tell me about a lengthy bicycle tour he was planning, much of it through some pretty remote places. Would, he asked, the popular Shimano Alfine 8sp hub be an appropriate choice under those circumstances? My answer is no, probably not. He suggested to me that, while he planned to stay mostly on paved roads, there was the chance that he would go offroad, and that he didn't want to risk mud jamming up his derailleur. Well, I ride offroad quite a bit, and I have had problems with mud jamming up my wheels, but I never had mud jamming up my derailleur. In fact, if mud somehow gathered around the derailleur, it most likely would also jam up the tiny, somewhat delicate parts that comprise the Alfine cassette joint, which is the external moving part that shifts the Alfine hub. Even if this wasn't a wildly contrived scenario, unlikely to really happen, I'd much rather wipe mud off a derailleur than try to extract mud from the recesses of the Alfine cassette joint...
Of course, when I hear "long bike tour", I also hear (in my head) "50 flat tires in the dark or the rain". With derailleurs and quick-releases and a little practice, fixing a flat is EASY and FAST and generally requires NO TOOLS (aside from the pump and maybe a tire lever). With the Alfine, fixing a flat is relatively difficult, time-consuming, likely frustrating, and is best accomplished with a 15 mm wrench, a 2 mm allen wrench, and a small flat-bladed screwdriver (plus pump and maybe a tire lever, of course). If I thought it likely that I'd have to repair more than one flat on a tour, I would trade all the alleged advantages of an IGH for the ease of fixing a flat on a derailleur bike. But maybe that's just me.