Hmm.
Well, although it's not considered especially useful on a relatively small engine, it's worked well enough on our Ka. The headline improvement is a shocking 3 PS, yes a whole three German horses, and 5 Nm of torque.
However, it flattens the torque curve, and post chipping the engine now produces >100% of original torque from 2,000 rpm to 4,500 rpm, with a gentle peak at 3,100 rpm. Across some parts of the rev range there's over 105 improvement in torque, which you feel.
The improved torque makes for improved flexibility, which translates into reduced fuel consumption and easier hill climbing.
Cue rant from Steve H about fuel consumption, torque, or something of this nature, heh.