I had a DS lite for over a year. A large libarary of DS games (20+). I could not get into it. I probably logged less than 50 hours on the DS total.
I traded for a PSP, about 3 months ago. I already have 100 gaming hours on the PSP. I probably play it more than my PS3, due to the fact that it is always close. They type of games for it though are generally closer to what you would find on a 360 or PS3. DS games, are like what you would find on the Wii. I think a large reason for this, is just the fact that I enjoy the type of games available on the PSP a lot more than the type on the DS. But that is just personal preference.
The web browswer is suprisingly good, and reading text is nice onthe PSP screen. But I find I rarely use it due to the text input being annoying.
It is a really nice music and video player though. If you have a PS3 it is even better, as you can stream video and music from the PS3 to the PSP. I have a media server (tversity) on PS3, which serves all my videos, and music to PS3, which my PS3 then streams to my PSP. No lag.
I also should add that the quality of the wireless hardware in the PSP (I have a slim, so not sure about the phat) is really good. In the far corner of my house, my dell laptop can barely pickup a signal, and when it does, it is around 30%. At the same location the PSP show 70%, and streams video fine.
~S