Mate i had the best time of my life on the Contiki tour. I can only talk about my experience as everybodies opinion and experience on contiki would be different. But i was lucky enough to be involved in a great group with a good tour manager. At the end of the day its the rest of the people that you're touring with that will either make it memorable and enjoyable IMHO. For first time travellers i think this sort of thing is well worth it. I decided to do what they call the "timeout" style of tour other than the cheaper "backpack" variety. Its a bit more expensive, but its more my style to be in a decent hotel rather than in hostels and sleeping bags.
The tour i did went for 17 days, and i knew we were going to be rushed through places, so if you select a short tour, just don't expect to see and do everything. I have no complaints of the tour other than i wish there was more time, but i knew i'd feel that way.
As for the Greek Islands, i went to Santorini and Mykonos. They weren't what i expected, not my style. Very dry, barren looking places...Santorini had crap beaches, Mykonos had decent beaches though, but i was hard pressed to see grass, trees wildlife etc.. I wouldn't go back in a hurry.
Cheers mate and good luck if you do go, i'm sure you'll love it too!