Okay, here's my opinion on the DS. I won't offer one on the PSP, as I don't own one.
It's a great time-waster.
I use my DS when I've got a few spare minutes here or there.
It's not designed as a system you'll be potentially spending hours with, and as such, the Mario 64 remake fails utterly on it.
But games such as WarioWare, Polarium & Zookeeper are fantastic timewasters. They're fast, addictive and fun. And quitting mid-game doesn't hold any massive ramifications.
Great for when you're waiting for the bus, or other such mundane stuff.
I believe this is one of it's strengths. Granted, this doesn't itself offer much more than the standard Gameboy or SP, but the use of the touchscreen & microphone just raises it above these prior machines.
It's something you can pass around to your mates, giving instant gratification using an intuitive interface that has little to no learning curve.
It can be uncomfortable if used for extensive periods of time, but I found the SP even more uncomfortable due to it's small profile. My fingers would cramp up quite quickly whilst using the SP, not really something desirable for a "Hand-held" games machine.
No such problem with the DS, although using it for traditional SP games using the D-pad and buttons leaves you with the definite sense that that's not really what it's designed for, but these buttons are the legacy of the DS' limited backward compatability.
Regarding the wide expanses of empty plastic, I honestly hadn't even thought of that until you mentioned it, and actually looking at it right now, I think your point here is invalid. There really isn't that much wasted space on the machine at all, especially when you consider the positioning of the D-pad and buttons in relation to the screen on the lid (I know, this is kind of contradictory to my last point regarding ergonomics, but if you
are going to have a degree of backwards compatability, they need to be there).
The screens themselves are both bright and equally clear, with no dead pixels, although I hear that the stories of prolific dead pixels on the PSP are mostly fistion as well.
Regarding build quality, I'll happily admit that it doesn't look the prettiest, and the silver colour does indeed make it look quite tacky.
HOWEVER, when you hold it and move the various parts, everything feels solid and well assembled.
Everything is tight, moving parts all snap together nicely, and the buttons feel solid, so whilst looking a bit tacky, it feels great.
I'm not happy with the finish on it. For a couple of days, I carried it in my pockets without any kind of casing, and as a result, a spot of paint has come off. The old plain plastic style casing would have been a lot more practical.
This does make it much less portable, as you pretty much HAVE to buy a case if you want it to retain what little good looks it does have.
The official Nintendo case is the size, and indeed has the look of a small handbag.
So as well as affecting it's portability, it's rather embarrassing to carry around.
I think Nintendo have got something unique in the DS, and are approaching it's main selling point rather well. Controlling games by touch adds a little more to the immersion factor, leaving you with a more tangible sense of having a physical effect on items in the games that take advantage of the touch screen. I think it's a valuable new addition to gaming (although something that could have been achieved on PDAs a loooong time ago).
Blowing or yelling in to the microphone is just plain silly though

Fun, but silly, and not something you'd really feel comfortable doing in public.
Time will tell with games on the DS, although initial sales figures are VERY good, and financially speaking, it's already considered a success story.
Now I hope people who are considering buying the DS will find this information valuable.
It's not based on the opinion of others, or the ravings of a fanboy.
It's pretty subjective, and considers the flaws as well as the successes of the DS.
And it's all based on first hand experience.
Sorry if I've insulted you Foo, but I hate people dismissing things they have little to no experience of.
I would LOVE to dismiss the PSP offhand, as I hate Sony as a brand, especially considering some of their more monopolistic business practises (one only has to look briefly into broadcast tech to get a feel for this).
I also am of the opinion that Sony's popularisation of the games market resulted in huge dumbing down, but I know that if I were to really sit down and be honest about it, it's probably been more beneficial to gamers as a whole than detrimental.
So I have to get some first-hand experience before I can really judge which is better. But before that happens, I'm going to read as much informed opinion as possible from sources I can trust so that I know a system is worth my hard-earned cash.
I would hope that other people would do the same.