I'd heard rumours that the receiver for this controller doesn't fit well with the Slimline PS2. Those rumours are completely false. It's snug against the memory card but not so that it's pushing against it. It's level with the bottom of the console, so there's no problem having the PS2 horizontal. When it's pushed all the way in there's actually a gap of about 1mm between the receiver and the edge of the PS2, which could lead people to think it's not all the way in.
As for the controller, it's very comfortable. It's a slightly different shape to the Dualshock 2, in my opinion, for the better. It has a smooth, matt finish which isn't rough on the hands but gives a good level of grip. The analogue sticks seem very slightly less stiff to me. In a matter of opinion, this could be better or worse. They certainly move smoothly.
I was a little unsure of the lack of a power button. But this is Logitech. I have a wireless Logitech mouse and it has great automatic power management, so I trust them with this controller. It turns itself off unless it has a connection to the PS2 and it automatically turns on when the console is turned on. If you press one of the buttons while it's "switched off" it will search for a connection for a few seconds. If it doesn't find a connection it will go back to off mode.
I've had no connection problems after a good while of constant use. One problem I've had with other wireless controllers before is that they sometimes randomly search for a connection in the middle of some play. None of that with this controller.
It's more expensive than most of the competition, but in my opinion it's totally worth it.