I don't have a PCMCIA sleeve per se, but I do have the CoPilot GPS system - which is a sleeve - and which has a slot for PCMCIA memory. I have a 128M card in it to hold my maps and some movies for my 3870. Seems to work good for me.
I've got both the 1 slot and the 2 slot for my iPaqs. The two slot sleeve is really thick. If you don't have two cards that you need simultaneously, I recommend the one-slot one. It still adds considerable thickness, but still feels contoured.
It seems to work fine. I use an 802.11b card in it for my wireless network.