I bought one on the day of release with Super Mario. Its a pretty nifty piece of kit, the touch screen is perfect and it feels good in your hands. The way they have utilised the dual screen is impressive too, and its not sluggish in anyway, it shifts those polygons like no ones business

. It has a built in rechargable battery which saves you alot of money in the long run. Also it has blue tooth built in so you can play against friends within a certain distance (say from 60-100 meters in the book). And there is Internet capabilitys for it, with a few online games comming out soon, which sounds interesting to say the least.
On the downside though, theres not many games out for it so far, that why I only bought Mario with it!
I think the PSP will be alot better, it has better graphics and much better game maker support, with around 30 titles on release! But im alittle warey of the battery life because of the PSP's big vibrant screen. And its going to cost £200, where as the DS is only (well i say only in a loose way) £100.
I think if you have the money for both you should definatly get the DS, and if you only have enough for one, I would go for the PSP without a doubt.