Have a look at the ST forum on this board, that'll give you all the lowdown on ST2's.
Similar power 85 Bhp ish output to the Multistrada, hasn't got the new heads and dual spark, but is water cooled and fuel injected. Just been discontinued in favour of the new ST3 with the 3 valve heads.
The ST4 with it's 105 ish Bhp 916 engine, and the ST4S with it's 117 ish Bhp engine are undoubtedly faster but for most roads, most of the time, the torque of the two valver will let you keep up with almost anything.
Here's a recent "should I buy one" thread [Edited on 8-12-2003 by Jools]