I got my silver PSP-2000 two years ago, and I've used it almost every day since.
It's my default MP3 player now (I have an 80 gig Archos, but I use that more for videos and storage) plus I use it to check the web, store photos and so forth. I used Skype a lot on it (with a separate headset) when I was back in Ireland, but don't need that feature so much while I'm here in Canada.
Also, game-wise, there are titles like Crisis Core, Dissidia, Syphon Filter: Logan's Shadow, Gran Turismo and others (like the three Fate games I have for it) which have had a fair bit of use already - plus titles like MGS: Peace Walker and Final Fantasy Agito XIII on the way which I'm looking forward to...
...and ones like Gundam vs Gundam Next Plus and Fate/Extra I hope to see picked up for Western release.
Plus, I like hooking it up to the TV and using it that way.
I'd say that if you wanted a PSP, I'd definitely suggest not treating it just as a gaming machine (though I believe it holds its own, for my use at least) but as an overall media device.
I'm not so sure on the Go, though. A smaller screen, new proprietary connections, no UMD use and the switch away from MemoryStick Duo add up to an option which doesn't appeal to me overly much... at least, compared to what I have already.