Make sure you get a good one and a decent bike cover. It wont stop a determined thief but will stop an opportunist thief. If you are unable or not allowed to use a ground anchor use the "bucket'o'crete" method. Large plastic bucket filled with concrete. Once set mount the ground anchor into that then chain the bike up. You would have to be really really keen on stealing a heavy bike chained to a very heavy concrete block.
A rape alarm mounted to the cover would stop opportunistic thieves, cover removed, pin pulled out of alarm, 120dB and scares the poo out of most people. Just dont forget it is there, dont ask me why I know that