If you're happy with your computer's performance then sure, I'd repair.
You could easily replace a DVDRW yourself, it's just a case of 4 screws, two cables and making sure the jumper at the rear is set to the same as the faulty one that's being removed.
Of course, if you really want to upgrade, that's up to you, but I'd say it's worth spending 30 to 50 pounds on your machine and keeping it for at least another year.
Depends, really, what you wanna do with it or what you're doing with it at the moment.