got immobitag on 2 bikes as I had vouchers for wiggle. cant say whether they work as (fingers crossed) they havent been nicked. They come in a plastic plug designed to fit in the seatpost but (1) they dont fit in large seatposts and (2) that only protects the seatpost not the frame! I prized the "chip" out of the mount and epoxy-glued it into a frame tube via the BB shell.
you can (and should) register your bike (and everything else) on http://www.immobilise.com. Many police forces do check this database to identify potential stolen property. In one custody suite I have visited every mobile phone brought in by a detainee is checked on the MEND database, a phone-specific subset of the immobilise database.
Crime recording databases are held by individual forces, so if your bike was stolen and recovered in a different force area (which might only be a few miles away) it will never be identified as yours unless it is registered on a national database such as immobilise.