Realistically, if a professional theive wants your car he'll take it, but the basic 3 point immobiliser will work as well as the person who fitted it.
The theive will automatically go for under the dash, if all the wires are exposed and he can easilly find the unit then your in a bit of trouble, thats when experienced fitters will make everything look standard and hidden to make it harder.
You will also want a backup battery siren thats nicely hidden so they don't just jam a cordless drill through it and shut it up (which i've seen happen before)
But a normal immobiliser/ alarm unit will deter the odd f**kwit that tries to break into your car.
Never tell people or have a sticker on your car showing what the unit is.
Hope this helps... good luck