I believe there is great value in updating reviews after an extended period of usage for, like you said, an initial impression does not take into consideration long-term durability/reliability or the more granular issues that become apparent with daily use. the CNET editors do update reviews for some of the most popular products to take these factors into account, so future purchasers may receive more insight than early adopters. However, there are two catches:
1.) Editors reviews potentially dozens of competing products. For example, an editor may review the iPhone 4, the HTC HD2, the Nokia N8, the Droid 2, the Droid X, a Blackberry, and more within a couple months. While someone may be able to review all of the phones thoroughly, it's not possible to continue using all of them on a regular basis. Thus, the number of follow-up reviews will always be limited.
2.) I believe CNET still has a policy of returning all manufacturer-provided hardware products after the review rather than letting the editor keep the product. The rule was created to eliminate both the appearance of and the potential for bias regarding a review 'being bought' in exchange for the product. Thus, editors would not be able to continue using the product for a follow-up review unless they actually purchased the device for themselves.
Keep an eye out for updates to product reviews, though; you just may gain some more insight prior to making the purchase.