Pros Large screen
8 megapixels camera
Camera shutter button
Cons Single-core CPU (Laggy Timescape UI)
Poor support (ICS took a long time, lack of JB)
No front camera
Impractical design (Audio port hard to connect properly, buttons hard to press)
Cheap build - plasticky.
Summary If you're a die-hard Sony fan, get the newer phones. Skip the 2011 series and move on to the 2012 ones. The arc S may seem to be a good phone at first sight, but after 2-3 months of use, you'll start to hate Sony (Ericsson) and this phone. Poor build quality, poor support, lack of front camera, weird placement of buttons and ports, mediocre CPU are only some of the many problems this phone has. Or rather, don't even buy a Sony phone, when you've got many other phones which have better specs (Quad-core CPU at least!) at the same price range.
Pros Screen is ok but the brightness sucks
Cons The Xperia's UI suffers from lag due to bugs
The screen is ok but the brightness sucks
Extreme lag issues with the browser
Bad 3G/4G transmission
Feels plasticky and low quality
Summary Bad phone with little functionality