You have to decide if the GPS functions are better for you than what your phone can do.
I got a GPS for my motorcycle because I can plan a specific route on my PC and upload it to the GPS (note that not all GPS' have this function).
Used Garmin on Ebay or at a flea market. Use google and figure out if the model has free map updates
I've read that free updates are only for registered GPS' (at least Garmin) and if the original owner doesn't un-register, no-one else can register it/them.
I got my Nuvi's (I ended up getting another one for my car) refurbished from Amazon (or Amazon re-sellers).
I did have to replace the battery in one of them (also bought from Amazon).