These models are either at the bottom of the depreciation curve or starting to tick up in value.
If you had told someone 20 years ago about the price of a used air-cooled Porsche 911 today, the person wouldn't have believed you. Where ratty mid-1980s 911 easily went for sub-$20,000 at the time, values today have jumped to double that amount or more. This slideshow might prevent you from feeling regret again for not buying a Porsche before the prices jump.
Some of these models, like the Boxster, have simply depreciated as much as they can. A drivable example of these roadsters is readily available for less than $10,000, and there's nowhere left for prices to fall.
Conversely, don't ever expect a 997-generation 911 GT3 to be so cheap. However, these cars give buyers fantastic performance and the possibility of making a few bucks when the time comes to upgrade.
Even if you get one of these cars, and the value drops in a few years, there are the fantastic memories of driving a Porsche.
Source: Auto Classics