Ask Engadget: Tips on picking a smartphone?

We received a ton of submissions for Ask Engadget this week related to the
PlayStation Portable (mainly asking us whether we think the damn thing is worth buying or not—how many times you can tell someone that it all depends on what you're looking for in a $250 device?), but we've sort of OD'd on the PSP, so instead we're going with a question from reader Eric S.,
who wants some advice on buying a new smartphone:

I have a question that I'm hoping you'll be able to use for Ask Engadget. I've reached the point where I want to
be able to do email and surf the web on my cellphone, so I think it's time to upgrade to a smartphone. I'm willing to
switch carriers to get a good deal, but I'm a newbie when it comes to this stuff and I need some advice about what
kind of smartphone to buy. A lot of people seem to like the
Treo 650, but I want to weigh all my options before I
buy something. Someone told me about that new
BlackBerry that's more like a
phone [Ed. note - He probably means the
7100t or the
7100g], but I heard that you can't
install your own applications on it, and that is very important to me. What should I look out for? How does anyone
pick one smartphone operating system over another? Is there anything else besides Palm, Windows Mobile, and Symbian
that I should be considering?

A tall order, does anyone have any advice for our new friend?