I think my previous couple of posts were a bit heavy handed so I've just decided to re-write my opinion in a bit of a kinder fashion.
I'll agree with what was said about the PSN not having all the games, and I'll also agree with the poor notification service. For example, I wanted to get Metal Gear Solid on my PSP, I scoured the net to see if there were any plans to bring it to the European PSN and I found nothing official so I went through the bother of setting up an American account and then buying the points to put onto said American account. Also, I had about 9 or 10 games on my regular European account and was solely using this American account to play Metal Gear Solid, which in it's self is an inconvenience seeing as you have to sign out and sign in each time. Sony then released it on the European store about 2 months later which if I'd known at the time then it would've saved me time and money.
What I will stick up for with the PSPGo is that it'll be tricky to find a PSP-3000 on the high street that comes with a 16GB memory stick and four full priced games for £190-200. It isn't for everyone though and when it comes down to it, it's pretty much whether you find the disks inconvenient to lug around. The Go is much more portable and the screen definitely looks much nicer when in use than the 3000. The screen is .5 inches smaller but it still displays games in 480 X 272, the same as the half inch bigger screen of the 3000.
It's 31 grams lighter, has integrated Bluetooth if that's a big deal to you for data transfer, you can also hook it up to a Bluetooth Dual Shock 3. The 3000 doesn't come with a data cable whereas the Go's power adapter also doubles as the data transfer cable, What happens is the cable can be unplugged from the plug, a bit like a shaver cable, one end plugs into the Go using a propriety connection type, and the other USB end plugs into the plug it's self. It's a nice little touch as it eliminates the need for two different cables.
The deciding factor when purchasing one should be whether you plan to carry a little bag for all your UMD's and batteries, how often you use it and where you use it, not to mention which you prefer the look of. For some people, having a chance to buy second hand games is a must, for other who aren't that bothered then the Go would be fine. Sure, not all of the games are up on the PSN, but it's hardly barren of any good games at all. There's Tekken 6, all the GTA's including Chinatown Wars, Gran Turismo, Assassins Creed, as well as all the PSone Classics that you can also download on PS3.
Maybe the Go is a tad pricey in comparison to the 3000, but it's definitely not a pointless device. Kaz Hirai was saying that it was designed to be an object of desire, it's basically a premium version of the PSP with download only capabilities so if that's your thing then buy it. If you prefer the option to buy second hand games and to carry extra batteries around with you then get the 3000.