I have used PSP for a long time and i am a fairly heavy user. I had problems with X3 using all patches so I could see my Sony SLT-A55 RAW files correctly but then Topaz Remask 3 would not work. I also had issues using Topaz plugins on 16 bit color. These are resolved in PSP version X4. The HDR lab seems to work pretty good and the additions of Fill light and vignetting are certainly welcome.
X4 seems to get sluggish when I have several 16 MP pictures opened and I am using a number of layers on one. I'm using a AMD 3.2 GHz PII quad CPU and 8 GB of RAM but an on motherboard ATI video chip. I didn't see this in X3 though. Overall I think it is worth the upgrade.