I've been testing Sony Ercisson's pint-sized Android phones, the Xperia X10 Mini and Mini Pro out for a few days now. Carried the Pro version with me all day yesterday and all morning today as my personal device.  A few quick thoughts:

• Despite the fact that both devices run Android, the combination of the small size, old (v 1.6) OS and custom SE software really limits the "full smartphone" aspects of the devices. You're just not going to do with these what you can with an Incredible or Droid X.
• That said, SE has done a very clever job with the custom UI. The four-corners homescreen is genius, and I love how you can scroll through widgets on a one-per-screen basis. Really maximizes the tiny 2.55" of screen space.

• The phones are tiny, but thick. The Pro version is a whopping 17mm thick.  That's almost two iPhone 4's worth of thick.
• THAT said, I went for a run yesterday and was able to tuck the Pro into the internal key pocket of my running shorts.  Pretty awesome.  I did 3.5 miles with the phone neatly tucked away, using it as a music player, and it was totally comfortable (the phone was pinned between my hip and the waistband of my shorts, so it didn't move around.

• The music player software is pretty limited. No playlist creation beyond auto-generated smart playlists. Or at least I couldn't figure out how to create a playlist.  So I put the player on All Tracks shuffle.

• Call quality has been good, but the phones don't work with my iPhone/Android compatible wired headsets. 
• X10 Pro's hard QWERTY keyboard is good, but not great. I'm thinking that the novelty of it will soon wear off, and I'll prefer the thinner, smaller, QWERTY-less mini to the mini pro.