OK, I bit the bullet and bought one of these - Cost me £150 notes.....
It arrived this morning.
First thing I noticed is that it doesn't sit very well on the PSP.
The bigger battery is ok - feels quite firm, but the HDD 'attachment' doesn't marry up to the psp well and it wobbles - it feels quite cheap.
The drive gets really hot too - I've copied about 3 1/2 gb of stuff onto it (which took ages).
Once you've booted the PSP back up, the access time is rubbish. The icons in the game menu take ages to display and the games take ages to load.
As you can tell from the pictures posted everywhere, it makes the PSP really bulky, but you can *just* stuff it into the original case with everything strapped to it.
Unless you are desperate for 4gb of storage, I wouldn't bother with this.
Wait for something better to arrive.
Just my opinion..... :)