I haven't read all the replies, but in response to the original question it's difficult one as you've asked it Full Frame is
worth the money over other cameras, and worth is a matter of opinion.
I've only got into photography relatively recently and struggled to tell a great deal of difference between a compact (a good example) and an APS-C DSLR. However, having gotten into photography more and started doing processing the difference between a compact is night and day.
Likewise, I couldn't see the difference between APS-C and Full Frame, but now there is a clear difference comparing images taken with the same lens. As pointed out already, the lens is arguably more important than the body, but there is a noticeable difference between APS-C and FF. I can't quite put my finger on what it is, but the images pop more (not just talking about DOF). They seem to capture more detail (un-related to MP), and there seems to be more 'depth' to the images. I will definitely be making the upgrade to FF if I can ever afford it.
What the difference is between mid range FF's and high end FF's is though I'm less aware. For example the Canon 5D Mark III can be had for around £2500, yet the 1D is £4500 and to my eyes there's no discernible difference in image quality to my eyes so I guess you're paying for things such as autofocus and features? It just so happens then when I search through flickr etc most of the images I'm wowed by just so happen to be taken with the Canon 5D Mark III, produces amazing images IMO.
I'm actually a Sony user (waits for the laughs to disappear, I have my reasons though

) and have the A77 (APS-C). When I compare images with the A99 (Full Frame) taken with the same lens (such as the 70400G) again there is a noticeable difference in image quality IQ.
APS-C's have 2 advantages though (which may have already been pointed out), the weight, and that they give a better reach due to the crop, so a 300mm lens actually gives you the equivalent zoom of a 450mm lens (480mm for some Canons). So if IQ isn't as important as reach then APS-C is definitely the way forward. But if IQ is the most important thing, and you have plenty of cash then full frame with a top notch lens is the way forward.
I don't know if this answered your question or not? To me the difference in IQ between my APS-C, which can now be had for under £700, and a £2k FF is worth the extra cash. However some will barely be able to tell the difference and so will struggle to justify the extra £1300.
One more thing worth noting is that although it's been said that the lens is arguably the most important thing, this isn't quite true. The person behind the camera is the most important thing. Someone that knows what they're doing can get a better image with a £200 compact that someone that doesn't know what they're doing with £4k worth of camera and lens.