You can safetly dump them. WordPress and some plugins will re-create transients as needed. A transient is more or less the stored value from a complex query. The results are saved as a transient so that the system doesn’t have to perform a common query over and over, instead it just looks for the transient if it exists and hasn’t expired. Of course, make a backup of your database before making a change lest something goes wrong!
After backing everything up, you can run a mysql statement like this:
DELETE FROM `wp_options` WHERE `option_name` LIKE ('%\_transient\_%')
[EDIT: statement fixed with escape characters, after comment suggestion]