Rim Cylinders are often referred to as Yale locks. These were fitted to most front doors before uPVC and composite doors became available. The rim cylinder is used with a night latch, this is the part fitted to the inside of the door.
The most common problem with these is when the door shuts behind you and the keys are inside!
They are relatively easy to by-pass so I wouldn't suggest a rim cylinder is the most secure lock. A lot of people also have a mortice deadlock, or sash lock fitted but don't use these as the rim cylinders are more convenient. I would advise you to use the mortice lock, if you have one fitted, along with the rim cylinder, as this will ensure your home or business is more secure.
If you have any concerns about your security or the locks you have fitted, give Pick Me Locksmith a call.