Most people know the main limitation of the PSP Go compared to the PSP-3000 (i.e. lack of a UMD drive), so I won't focus on that in this review.
The PSP Go is a really well built piece of kit. The screen is slightly smaller than that of the 3000, but it has the same resolution and looks really nice. I have fairly big hands, but even though the buttons are quite small, I had no problems at all when playing games. The battery can be charged using the supplied charger or via USB (a proprietary cable unfortunately, but it is supplied) although you cannot charge the system via USB while actually playing games. Compared to the 3000, it fits nicely in a jacket pocket.
The features have been covered by the other reviews - you've got games, movies, music, RSS feeds, Internet radio, web browsing, remote play from a PS3 and more.
There's a decent choice of games on the PSN store - full PSP games, PS1 titles, some demos and PSP Minis (cheap, smaller titles like Tetris). It's a shame that Sony haven't made it mandatory for developers to offer a digital download version of every game coming out since the Go's release, as there are still titles coming out that are UMD only. All first party titles will appear on PSN though. Downloads were pretty fast via wi-fi most of the time, although games also need to be installed which can also take quite a long time. You're not going to be buying games daily though, so that's not really a problem. If you have a PS3, you can download games with that and transfer them over to the PSP with the supplied cable. You can also use Media Go via a PC to access PSN, although I found the software quite bloated and it adds half a dozen entries to add or remove programs in Windows as it has to install Quicktime and several other things.
As for the negatives, it's a shame that Sony didn't stick with a mini-USB socket for charging and transferring content to the system. Also, the 16 GB of memory is actually 14 GB as that's how much usable space you have for games and other content.
I paid 149.99 on sale at Play for mine, which I think is a good price when you consider that there's 16 GB of flash memory built in which would cost you about 45 pounds to buy. I've knocked a star off for the proprietary USB cable, the lack of some new titles coming to PSN, and the clunky Media Go software, but as long as you realise that this is a digital download only system for games and take into account the limitations that come with that, I think you'll be very happy with your purchase!