With any new product it takes awhile to work out the bugs and problems. This new iPhone has a number of significant, and largely positive, changes. However there are always risks associated with major changes.
3G phones have had historically poor battery life; this is one of the reasons Apple was slow to bring one out. There has been no magical solution to the limitations of either battery life or power consumption when performing at full 3G speeds. And Apples strategy of stepping down the radio could do some really painful things to applications. The iPhone will eventually start dynamically shifting bandwidth to conserve power, but this feature is not yet being shipped so application vendors cant test against it. Once it does ship, applications that use this bandwidth are likely to either not perform well or prevent the power conservation feature from working initially, until they can be rewritten. It will take awhile to work though this. The improvements promised in September should address most of these issues.
The presence of 3rd party applications may also have an adverse impact on reliability and you may want to move slowly in terms of loading those applications regardless of when you buy the phone, so that others experience the problems first and you benefit from their work as opposed to the other way around. This experience should improve dramatically over the next several weeks.
Finally, they have cost-reduced the case of a phone that has a large glass screen. Plastic may actually hold up better than the aluminum case that the old phone has but it will take a number of weeks to confirm this and, if it doesnt, Apple will likely fast track a fix. While the fix probably wont make it in September, we should be aware of any problems by then and be better informed with regard to how to protect the phone.
Im not suggesting you dont buy the iPhone, just suggesting you wait until you can make the most informed decision. This year in particular money is tight and this phone will cost you nearly $2,000 over its life. If you find youve made a mistake youll have a lot of months to regret that mistake so, Im suggesting, it is better to ensure the decision is the right one than it is to be one of the first to get the new phone.
Or, putting it another way, do you want to be first or appear foolish? The folks who initially stood in line for the first iPhone ended up feeling foolish. You may want to learn from them.