It is definitely worth it if you intend on keeping the car for another while yet and providing nothing much else goes wrong with your car. If the cam belt goes then you could as good as write the car off. The cost of repair if that happened would be massive.
Just a piece of useful advice re cambelts:
If you buy a used car, check when the cambelt was last replaced, or get someone to inspect it.
If in any doubt, GET A NEW ONE FITTED. �100-�150 typically is a small price to pay compared with a possible four figure sum for repair if it fails.
I had a cambelt fail on me 17 years ago and it cost about �500 then to repair my poor car...NEVER AGAIN !
Make sure that the replacement belt is a Renault part and not a "pattern" part. Also make sure that tensioners etc are replaced. It should all come as a kit.