I run Final Cut Pro, MacroMedia suite, and various Adobe applications on the iBook in my sig. If I were really editing video on it all the time, it would get annoying--but it is bearable.
I suppose the question is, what do you want to be able to do with it? My iBook is primarily for web, mail, IM, and MS Office type stuff. As a grad student, it has been indispensable--especially with the airport extreme card allowing me to surf the web wirelessly in the library. Despite some who complain of "screen real estate" issues, or eye strain...I had no problem writing my entire thesis on the machine...but, not to brag, I have quite good vision!
So, if you want to do stuff like that, then you're fine with 800mHz and 512MB RAM...if you want to get into some photoshop or final cut or games...then you might find the iBook wanting.
Finally, the 30GB hard drive is a little puny...might wanna look at externals (or internals with enclosures...which is what I've done...might save you money depending on whether or not the seller has some deals workin')