Have one in my bike - I got mine fitted free by the local police.
My local force is currently advising the recording of frame numbers, and property marking using UV pens instead - although they have the readers for the immobitags, they're relatively expensive, and they aren't carried by PCs on patrol. (They can check the immobilise database for frame numbers, and UV marking whilst out and about, immobitags can usually only be checked at the station).
I'd have a word with your local police and ask for advice - if you do get an immobitag, record your frame number and UV mark the bike as well.