Use this tool to check the coverage:

http://compass.t-mobile.com/ »

U cant compare it with cingular(growing) since tmo has a large coverage area in HI. When we went down there my buddies cingular phone was roaming on tmo most of the time...here is more feedback that would help u
http://www.howardforums.com/showthread.php?t=394537& ... »
http://www.howardforums.com/showthread.php?t=542728& ... »

T-mobile guaranteed the quality of service in HI for several years, meaning if you signed a contract under the "Hawaii Guarantee" as it was called and were not happy with the service you were free to cancel at any time without penalty. I think that speaks volume about their coverage in HI. The guarantee is no longer available to new customers but the service remains the same.

The T-Mobile coverage on Oahu is Good. I had it for a couple of years. You will have trouble inside some buildings, meaning no coverage. I switched because I got tired of going to the Mainland and not having coverage and also because of the customer service or lack thereof. I picked up Verizon which most people say has the best coverage on Oahu and I also have unbeatable coverage on the Mainland. I didn't go with Cing because I talked with many coworkers that were unhappy with them and the former AT&T.

If you want decent coverage on island for a couple of years and don't plan to travel or don't care about customer service I guess you can go with T-mo.

I personally know that I have better coverage with Verizon that t-mo on Oahu.