Well, I haven't actually used it yet, but I installed it last night and figured that I would pass this on.
I make it a point not to install applications on my C: drive. Most applications will allow you to pick another drive to install to without a problem.
Well, when I tried to install PSP X2 last night, I pointed the app to my E: drive (which is where I install all my apps). I then pushed the button to continue, but it complained that it still needed over 300 MB of free space on my C: drive!
It took me about an hour to clear enough space on my C: drive for the install. Not very user friendly or extensible.
Once I finally got it installed, I did manage to open the app and take a quick peak. Looks nice from a UI perspective, but that was all I had time for.