Any alarm is better than none and gives added piece of mind (I went for a toad on mine), I wouldn't bother with the original Conlog or Scorpion alarm as they are unreliable and not worth the hassle (both were a new "dealer" option but fitted after production in the UK)
I get 10% off my insurance with mine (so £60 a year) and will of paid for itself by next year ( mine cast £180 2 years back and it's a no brainer if your keeping the car)