Okay so recently I bought an Arduino to try it out. To be honest I find it a bit boring after the PIC because it is very simplistic, there's no real challenge. I like to know what the code I write does, I think you learn more that way. In the case of the Arduino the chip has already been programmed with some kind of boot loader so you just have to write some simplified code for it to run. Now don't get me wrong, it is extremely simple and makes project really really simple so if you're not interested in the actual working of the chip or you just want results quickly it is a brilliant tool. I just like the satifaction of knowing I wrote all the code even if it doesn't work anywhere near as well :)
The only other issue I have is the pin positions for creating shields, I got an Arduino UNO and in my case the pins don't easily line up with 2.54mm spaced strip board. I don't know if its a design flaw or intentional so that you're forced to buy Arduino shields instead of making your own or then again mine could just be flawed either I don't know, it seems strange to me.
That said it is a great tool for making complicated projects very quickly and there is so much example code you couldn't possibly get stuck for long. So hopefully I'll get adding my experience of a few different projects soon enough.