I guess I'll just chime in and say that, going into my fourth year, I've never felt frustrated/isolated/depressed more than I thought was normal. (I mean, does getting frustrated with GHC count? :P)

I'd actually say it's the opposite: the process of getting my PhD has been one of the more wonderful experiences of my life thus far. Although, I tend to believe that at least part of this is due in large part to one particularly awesome professor that I've worked closely with (and isn't my advisor).