I'm in Tonga at the moment, go to Samoa next and have just came from the Cook Islands and so far I'm totally loving the South Pacific. I spent two weeks in Rarotonga but if I was booking my ticket again I would have stayed longer. I went to Fiji last year as well and that was really good as well.
magykal1 is spot on about flights. If you book individual flights once you're out here then it tends to be exepensive (although Virgin Blue is expanding its network so flights may get cheaper in time), so your best bet is to get as many stop-offs on a RTW ticket as possible.
When I booked my ticket I had to fly to the Cook Islands and then on to Tonga via Auckland as the Cook Islands weren't directly on the Tonga/Samoa route, so this took me into the next miles bracket which cost me an extra 70 pounds, but it was still much cheaper than booking separate flights and totally worth it.
