It's successful itself, but not that successful for its users, though. One of the chronic problems of Arduino is that many people become mentally attached to it and exhibit extreme reluctance to ever move beyond it, learn how it really works, or (gasp) build a project that isn't an Arduino shield. When they find a problem that the Arduino isn't great at, they go to great lengths to solve it while keeping the Arduino (using e.g. more external logic, more than one Arduino, or, in extreme cases, throwing an FPGA on top of the Arduino - and implementing a coprocessor on it that is more powerful than the Arduino itself).
Very often, these problems can be solved much more efficiently by building your own hardware from a bare microcontroller, which is actually a very easy task - the Arduino is little more than a breakout board. Alas, while some Arduino users do realize this (and use bare AVRs, both the ones on Arduinos and other models, or even other brands, to great effect), most do not. Now that Arduino is moving to ARM, this will get marginally worse - if people aren't taking the plunge and figuring out how to use a bare AVR (which is completely trivial to bring up on a breadboard), moving to a surface-mount part that has slightly more complex requirements is not going to make things better.
The same thing happens, to a different extent, on the software side. I've seen many projects that are coded as a humongous unreadable Arduino sketch, when by that point the authors really ought to have learned how to modularize their code (and format it properly, too). It doesn't help that the Arduino IDE isn't a particularly great text editor.
The Arduino is a great learning platform, but it does the "make it easy" experience so well that people are very afraid of moving beyond it, and this ends up creating an artificial learning curve further down the line that is hard to get past. If you're forced to experience bring-up of a micro on a breadboard from the first day, you quickly figure it out (it's really easy), but if you've been comfy in Arduino land for months/years, it feels a lot scarier than it really is. (What do you mean I have to provide my own regulated power supply? And the pins, they don't have labels! Where is the USB connector?). I don't even blame the authors of Arduino; for example, they do include code that can program a bare AVR using an Arduino as a programmer (with visual wiring diagrams even, IIRC), which is exactly the kind of thing that you want to help people bootstrap themselves, but people still just aren't doing it.