I live on Saipan, but go to Guam numerous times a year for diving/shopping/transit. I can attest to it being a great hopping off place to hit other destinations. All Continental roads lead into and out of Guam, so fares are "cheaper" if you fly out of there and you don't have to deal with the connections we have to from other Micronesian islands. Plus you get special rates on airfare, food and other goodies if you have a local driver's license. I've been to both Guam and Hawaii (but never lived on either), and I thought that the anti-mainlander sentiments were stronger in Hawaii than in Guam. Tumon Bay is a scaled down version of Waikiki. I didn't like Guam until I started diving there and visited the southern part of the island, which reminded me very much of the north shores of many of the Hawaiian islands -- laid back, friendlier people, lush, windswept, rural. Diving is better than in Hawaii, but you won't see the colors, pelagics or macro action as you do in other places (Palau, Philippines, Indonesia, PNG). Diving is also CHEAPER than in Hawaii, especially if you join a local diving club.
www.mdaguam.com is a good place to start. They offer a discount card that pays for itself the first time you dive or buy anything of value. Guam Tropical Dive is an alternative but I've never dived with them, just had some of my regs serviced there (they did a great job).
Note that many people I know are moving away from Saipan and Guam, and the papers report a larger than usual number of bankruptcy filings, foreclosures, and businesses for sale. The economy in this area, although improving, is still depressed. It's driven by the Asian tourist market, mostly Japanese, so those economies are intricately tied to the region's. We took a heavy hit in the region after SARS, and Guam is still trying to recover from the physical and economic damage of Typhoon Pongsona back in Dec. 2002. But don't let this news discourage you. It's a great time to buy property if you're willing to wait for the next business cycle peak.