The Satio is a good S60v5 phone, but S60v5 really was half baked. Actually, I think it's probably the best of class for the older Symbian architecture. It, along with the Samsung Omnia HD, where the first to give S60v5 the 256mb of memory it needed to function correctly. So, unlike the 5800 or N97 which struggled to run more than a few applications at once, the Satio is a very comfortable device to use. Once you hack it, you can also run 80% of the proprietary Nokia apps like SportsTracker and Nokia Maps (I even had the Ovi suite of apps, including the Ovi Store, running on my i8910 after it was hacked).
That helps a lot, but there are two problems: One, S60v5 is dying. There will still be Nokia phones released with the older OS for a little while, but it really is on its way out. The plus is you can run many S60v3 applications just fine and that takes some of the sting out of it. The other is that Sony has said they're completely done with Symbian. That means support on the company's side is completely gone and you'll likely not see any updates from here on out for it. Of course that's not the end of the world either. The only real world gripe I ever had with the Satio (and most Sony devices) is it doesn't have a 3.5mm headphone jack and uses a pop-port instead.
I think if you're looking at the Satio you would definitely need to look at the N8 as well.
Here's a side by side comparison, but as you can see the N8 basically has the same internals as the Satio, but improves on many of the specs. It also has the newer version of Symbian that is sure to be around a lot longer than S60v5. If they both cost the same, I'd get the N8... Honestly, though, I prefer Sony to Nokia just due to build quality and how much better the desktop experience is. Nokia PC Suite is still way too bloated. Sony's desktop software is pretty damn nice. It's funny how much that matters, but when you're constantly syncing data and etc, you'll want a good desktop package.
Anyway... yeah... the Satio is probably the best of the S60v5 crowd, has a great camera, and I'm sure would make a really good pocket companion.