I previously had a SE P990i which this model replaced.
The main differences on the plus side between this and the P990i are that this has a more powerfull processor, a much stronger steel frame, a longer stylus, seems to be quicker downloading internet pages, has full today details on the home screen, has a 3.2 mp camera, and is smaller and lighter. There are no real down sides although the screen is slightly smaller (2.6 inches as opposed to 2.8), and the qwerty rocker keyboard takes a little getting used to, but is ultimately more comfortable to use.
Like the P990i the Quick Office software included is fine for viewing and updating excel and work doc's that are not huge (in word terms abour 28 pages). If you wish to work on very large documents I found that Documents To Go can cope with documents 100 pages long (this is 3rd party software that I purchased seperately.
Other 3rd Party software I use is the SlovoeEd dictionary (works fine although generates a message everytime you shut the software down), and Handy Safe (secure passwords). Sony Ericsson Media Studio 3 is good for converting DVD Films to your phone. Google Maps works fine although the version I tried to download from the Sony Ericsson web-site would not pass the security check (I simply downloaded a older version from elsewhere).
The P990i was an excellent phone once the firmware was updated. This phone seems to be fine without firmware updates (right first time!).
Top Tips- The hardware back key doubles up as a key lock. Simply keep your finger on the button to lock/unlock the device.
My 8GB memory card works fine with this phone.
I downloaded the xjaguar blue theme (free) from the internet which I personally feel works very well.