If I may jump in here...
As a new PB360 owner, I am completely prejudiced towards it, so you may have to take my comments with a grain of salt.
But, along with all the excellent points already mentioned, I think there are three good reasons to get it:
1) The ingenious cover! No other device has a built-in cover like it and this makes the PB360 a truly portable device, in that the cover protects it perfectly. Plus, it doesn't add much to the weight, it clips to the back while you are reading it and it is hard plastic. Imagine carrying around a CD case. That's about how big the PB360 is. It fits into the pocket of my trousers, shorts, etc. All with hardly a care because I know the cover will protect it.
2) The ergonomics. I like having, essentially, nothing but screen. With the buttons all on one side, there is nothing else to the device but the 5" screen. And, as the buttons are programmable, you can set them up to operate almost any way you choose. As the PB360 does rotate (360 degrees, ha ha), the buttons rotate with, so you don't have to remember which button does what. They stay the same. I currently have four buttons programmed to do nothing but go forward (the LH and RH side of the silver button and the 2 wing buttons) and I constantly switch my PB360 [easily] from my left hand to the right. Very comfortable to hold and use as essentially, the buttons are anywhere you want to hold it.
3) The versatility. Between the company provided firmware and then the ability to hack it (FBreader180, the Adobe hack, etc.) have made this one of the best things to use to read darn near everything. Granted, it does not do well with PDF - but I only read text novels 99% of the time - and reflow doesn't do that bad. Then there is the folder management system, the themes (to customize your own views), etc. I could go on forever ...
Yes, there is a click when you press the buttons - but not as loud as it sounds in any video. However, I don't notice it when I am reading (and I've already read 20+ books on this ... or more). Actually, now that I think of it, I LIKE that the buttons are not so sensitive and that I can tell I've pressed a button. Less confusion for me...
As always, this is my personal opinion. But, as a recent joiner of the Mighty Midget/Mighty Mouse/PB360 club - I am happy with my choice.
Good luck to you in your search.
P.S. And, I read on the 5" screen more than I read on my Irex's 8" screen. You just have to press the page turn button more often (I've enlarged my fonts, which I've selected for myself, thanks to the PB360 programming). Honestly, I don't notice the screen difference once I am engrossed in the book.