I have been using the iPad constantly since the 27th may, it is great, fast and a pleasure to use, I run backgrounder and usually have Skype, msn, itap rdp session and ministry of sound radio running in the background, I almost don't touch my work laptop anymore as I am able to do most of my work on the laptop via the iPad using rdp, this works great especially when out and about or working from the pub. My only gripe is that when I browse some large web pages or run a game, the memory gets low and sky or itap or radio would close, the 256 mb of ram is simply not enough and that is a shame cause the pad is fantastic.
I have a 3GS when it was on 3.1.3 it would have 175 mb of ram when you turn it on, since upgrading to ios 4 I noticed that 3GS now has 140 mb of ram available, unlike the iPad, I don't come close to using up all the ram on the iPhone.
ios 4 on the iPad is going to come at a cost of 35 mb of useable ram I will be very reluctant to upgrade.