I've never done Contiki as such but I've done a similar tour. I've also done a LOT of independent travelling.
Forget what the travel snobs say; Contiki type tours are awesome fun. You'll meet heaps of great people, see some wonderful places, have some unforgetable experiences and the memories will stay with you for years. Sure, you only just "touch the surface" of many places, but so what? At least you get to see them and cover a lot of ground in a short time. It's better to see a place for a short time than not to see it.
If time is limited and you want to cover a lot of ground and have an awesome time, do the Contiki tour. You won't regret it.