Underpowered by modern standards? Not really, 150bhp from a normally aspirated 2.0 litre is still a reasonable benchmark. The point is that Alfa got there well ahead of the pack (with Honda). Most manufacturers have realised that 150bhp is the practical maximum for a mass produced 2.0L engine that has every day useability. Most have had to turn to turbos to take the step up to 200bhp (Alfa included and again Honda excepted). Finally in terms of BHP per litre the twinny is identical to the 3.0V6 if I am not mistaken.I think they are underpowered (by modern standards), lacking in low rev torque, fragile and thirsty.
Lacking in low rev torque - no point in denying it and kicking it down a gear smoothly needs some degree of skill as the clutch is so light, but you either delight in this aspect or you don't. Any half decent diesel these days will nail a V6 on torque though, so V6 owners should n't be too smug about this one.
Fragile - relatively yes, its not that they don't thrive on hard driving its just that owners don't warm them up or maintain them properly.