We had an iPad at launch, sold it, but I'll likely buy another at some point. We also own Macbooks, MBP's (several) and iPhones (3G and 3GS).
There is NOTHING you can do with an iPad that you can't also do with either an iPhone or a Macbook/MBP. Its just that you'll do it differently and, depending on your perspectives, perhaps better.
For example, you can read a book on an iPhone and a Mac but the iPads book reading experience via Kindle is very similar to actually having a Kindle. Instant on, easy to use, good screen, simple touch controls. Makes it pretty comfortable to use in a variety of positions. Compared to the iPhone (screen very small) and the Mac/MBP (gets hot on your lap, heavy, trackpad navigation), the iPad is clearly the better experience. Same goes for watching a video.....the iPhone/Mac/MBP can all do what the iPad does but the iPad makes this a much nicer experience.
Web browsing for non-flash sites....the iPad is very easy and fun to use. And the fact that it has such a long-lasting battery means you'll use it for that and not worry about burning out your iPhone battery. I seldom use my iPhone's browser for extended periods as is as it is my primary business line and I NEED my phone. I only get 5 hours of use from the device just from talk time alone. You won't worry about the battery life on your iPad. It can easily go 2-3 days between charges for normal use.
Apps that require precision user interfaces, I'd still give the Mac/MBP the clear vote....the iPad can do OK with most content creation, but its not optimized for that. Its primarily optimized for content consumption.
I'd argue the games on an iPad are better than those on a Macbook Pro simply because you can play them anywhere, minutes at a time, standing on a bus/subway, waiting in a docs office, etc. Games are obviously richer and more powerful on the Macbook Pro but you aren't likely to whip it out for a quick gaming session at the doc's office in the waiting room. Its just too unwieldy for that.
I do like the "discreet" value of the iPhone for game playing, catching a quick video clip or even quickly grabbing a few pages from an ebook. It's small, fits in your pocket and goes everywhere with you. The iPad is too big to not call attention to the fact that you are using it. I was at my daughters' choir concert the other night and there's a 5-7 minute wait between singing groups as the kids filed off the stage to be replaced by new ones. I'd use those 5-7 minutes on the iPhone to catch up on email (read and quick replies) and not disturb or bother anyone in the dimly-lit concert hall. Doing so on my iPad would have quite frankly irritated those seated around me.
There are those who believe the GPS functionality in an iPad, simply because its a bigger screen, is worlds better than on an iPhone. I think the iPad is way too big to function as a GPS device in the car....it would distract me. YMMV. And walking around using one in a big city is just plain dangerous as it sends out the clear signal that you are lost and carrying an expensive bit of kit. I'd have to give the iPhone the win here because functionally it works well and its always with you...you often don't need a GPS until you are lost and thats not the time to wish you had brought your iPad while your iPhone is always with you.
For that reason, I can safely say I'll be getting at least one 4G iPhone when it is released and I just last month purchased a new 2010 i7 MBP. Figure on another iPad in our future and I can still find room for all three.
In summary:
Web browsing (non Flash sites): iPad wins
Book reading: iPad wins
Video watching: iPad wins
Content creation: Macbook/MBP wins (and some form of iTunes host is REQUIRED to even use an iPad so thats a no brainer).
Ultra-portability, jack of all trades, master of none: iPhone wins
Phone/Text: Duh....iPhone
