I finally got to mess around with one for an extended period of time. My thoughts:
-Slick, very slick interface. Browsing on this thing is pretty pleasant. Lack of Flash WAS noticeable in my limited online travels.
-iPhone apps scaled up to full screen size look pretty shitty
-"native" iPad app reformatting by Apple is well done. E-Mails in particular are excellent.
-iBooks is also very slick, but I'm not so sure about reading for a long period of time on this device's LCD. I did a little speed reading and was already feeling the pangs behind my eyes, just slightly.
-the weight, referenced in the OP is indeed an issue - my arms were getting tired after a few moments of holding it up. This thing works a lot better when it's on an incline on a table, or, maybe as the oft-mentioned "couch device" resting against your leg or whatever. It's not comfortable using this thing while standing or walking without a surface of some sort. Both because of the weight and of course that it's too big to be one-handed and palmed like an iPhone. For instance, you wouldn't be able to comfortably walk down the street and check your e-mail. It's just awkward unless you're sitting.
-Typing in notes on a landscape-view keyboard was more accurate than I thought it would be, but doesn't match the laptop experience. Also, impossible to type without a surface. People with LARGE hands may be able to thumb type in portrait, maybe.
-Apple's case for it is quite nice and bends back to give the iPad an incline on a surface, which does a lot to help it's usability.
The mantra for all Apple products should be: "buy the 2nd generation".
The 2-Gen device is always a rather large leap from the 1st, fixing a lot of things wrong, while the 3-Gen is usually just a small incremental improvement on the 2-Gen. (see iPod Touch adding speaker and upgrading the design, the 3rd Gen was just a size refresh, same with iPhone to 3G to 3GS). I expect the 2nd Gen iPad will be a rather spectacular product, especially if the iPhone 4.0 OS rumors are true that they might introduce some multi-tasking capability...
As of now, it's slick, but awkard. The weight, form factor, and sitting vs standing vs table all combine to make it kind of an awkward device. You basically need a bag to carry this around, like you would a laptop.
Personally it just doesn't make a lot of sense for someone like me, who has an iPhone and a laptop, and is also a power user who does a lot of writing and image creation and all that stuff. But it's a generation or two away from being a very good net-book replacement. Uh, as long as you don't have to do any real long-form writing.