The browser on a PSP is okay... it certainly isn't as robust or usable as the browser on an actual computer, of course. Probably the biggest drawback is that there's no keyboard, so entering text isn't fun. However, they do have a text entry system that is a little like text messaging on a cell phone, so entering URL's, logins and passwords is made a little less painless that way. I'd say browsing message boards is fine, but you probably won't want to post much.
I have a DSL connection at home with a wireless router, and images load pretty quickly. Maybe just a touch slower than on my PC, but pretty close. I rarely come across a website with any kind of formatting problems on the PSP browser.
Also, if you update the firmware to the newest version Sony offers, 2.8, there's an RSS subscription function where you can set it up to auto update anything with an RSS feed. This means podcasts, video podcasts, news sites, blogs, etc. In fact, the 1up Show comes out in PSP format every Friday in addition to the regular QT and WMV files. Be advised, however, that updating to 2.8 will mean no homebrew for a while while the homebrew community catches up (2.8 was just released a few weeks ago).
Finally, there's a PSP site that is specificly formatted to the PSP's dimensions. On it, they have demos, trailers and product news.
Here are a couple of links...
http://www.psp-vault.com/
http://www.pspfanboy.com/