Views on alarms vary. Most newer bikes have an immobiliser as standard.
Alarm wise there are 3 options;
1) don't fit one
2) fit one yourself/get a mate to do it
3) get a specialist to fit one
I would have one on a new bike, price dependant. I wouldn't retro fit one. I would not be put off buying a bike if it had no alarm. My SV has a Datatool and it has never caused me any problems.
I would look at Datatool for a specialist (get a certificate for slightly cheaper insurance) - £400.
I would look at www.cyclonealarms.co.uk for retro fit alarms as they seem to be good value and a good reputation - £35.
I would buy a big chain anyway but make sure you use it. Alarm cost - £0
I do not know any decent data to say how effective an alarm is, or isn't. Bikes do still get nicked if alarmed but who knows if an alarm puts off a big percentage of would be bike thieves?