I think the state of the developer builds are very telling of the overall readiness of a software product. When a product is in development, there are very clear signs as to what stage it's in. Currently, each new developer build of Leopard has been adding features. Typically, when a product is just weeks from release, features are no longer added and the final product is just refined and stabilized. From what little I've been able to glean from the reports on the last developer build, it's clear to me that they haven't gotten to that stage yet.
For those of you new to the Mac, understand that OS X is not changing in any fundamental way with the release of Leopard. There will be a few new features, but everything is evolutionary, not revolutionary. Tiger is still light years ahead of Vista in every appreciable way - and although it's nearly 2 years old, there's nothing lacking in it that can't be solved with aftermarket applications.
I've said it before and I'll say it again, if you're waiting to buy a computer for "the next big thing", you'll be waiting forever. Technology changes constantly and with Apple as tight-lipped as they tend to be, there's no way to know for sure when a new product will be released. So, if you like what's out there at present and it suits your needs, just buy it. There's no reason you can't upgrade easily to Leopard when it eventually does appear.